add missing virtual destructor in base class SocketServer used by TCPSocketServer and UnixSocketServer

[matlo607]

I got this error while building cucumber-cpp.

In file included from /home/matt/.conan/data/boost/1.66.0/murex/testing/package/c4cf4727ec6abec83cfac5a4693c657b37ece27d/include/boost/checked_delete.hpp:15:0,
                 from /home/matt/.conan/data/boost/1.66.0/murex/testing/package/c4cf4727ec6abec83cfac5a4693c657b37ece27d/include/boost/smart_ptr/shared_ptr.hpp:26,
                 from /home/matt/.conan/data/boost/1.66.0/murex/testing/package/c4cf4727ec6abec83cfac5a4693c657b37ece27d/include/boost/shared_ptr.hpp:17,
                 from /home/matt/.conan/data/boost/1.66.0/murex/testing/package/c4cf4727ec6abec83cfac5a4693c657b37ece27d/include/boost/date_time/time_clock.hpp:17,
                 from /home/matt/.conan/data/boost/1.66.0/murex/testing/package/c4cf4727ec6abec83cfac5a4693c657b37ece27d/include/boost/date_time/posix_time/posix_time_types.hpp:10,
                 from /home/matt/.conan/data/boost/1.66.0/murex/testing/package/c4cf4727ec6abec83cfac5a4693c657b37ece27d/include/boost/asio/time_traits.hpp:23,
                 from /home/matt/.conan/data/boost/1.66.0/murex/testing/package/c4cf4727ec6abec83cfac5a4693c657b37ece27d/include/boost/asio/basic_deadline_timer.hpp:28,
                 from /home/matt/.conan/data/boost/1.66.0/murex/testing/package/c4cf4727ec6abec83cfac5a4693c657b37ece27d/include/boost/asio.hpp:24,
                 from /home/matt/.conan/data/cucumber-cpp/0.4/murex/testing/build/232742180d6b48ffcaf9de5114f5a9b14f6df7e4/cucumber-cpp-0.4/include/cucumber-cpp/internal/connectors/wire/WireServer.hpp:8,
                 from /home/matt/.conan/data/cucumber-cpp/0.4/murex/testing/build/232742180d6b48ffcaf9de5114f5a9b14f6df7e4/cucumber-cpp-0.4/tests/integration/WireServerTest.cpp:1:
/home/matt/.conan/data/boost/1.66.0/murex/testing/package/c4cf4727ec6abec83cfac5a4693c657b37ece27d/include/boost/core/checked_delete.hpp: In instantiation of ‘void boost::checked_delete(T*) [with T = cucumber::internal::TCPSocketServer]’:
/home/matt/.conan/data/boost/1.66.0/murex/testing/package/c4cf4727ec6abec83cfac5a4693c657b37ece27d/include/boost/smart_ptr/scoped_ptr.hpp:88:30:   required from ‘boost::scoped_ptr<T>::~scoped_ptr() [with T = cucumber::internal::TCPSocketServer]’
/home/matt/.conan/data/cucumber-cpp/0.4/murex/testing/build/232742180d6b48ffcaf9de5114f5a9b14f6df7e4/cucumber-cpp-0.4/tests/integration/WireServerTest.cpp:84:7:   required from here
/home/matt/.conan/data/boost/1.66.0/murex/testing/package/c4cf4727ec6abec83cfac5a4693c657b37ece27d/include/boost/core/checked_delete.hpp:34:5: error: deleting object of polymorphic class type ‘cucumber::internal::TCPSocketServer’ which has non-virtual destructor might cause undefined behaviour [-Werror=delete-non-virtual-dtor]
     delete x;
     ^
/home/matt/.conan/data/boost/1.66.0/murex/testing/package/c4cf4727ec6abec83cfac5a4693c657b37ece27d/include/boost/core/checked_delete.hpp: In instantiation of ‘void boost::checked_delete(T*) [with T = cucumber::internal::UnixSocketServer]’:
/home/matt/.conan/data/boost/1.66.0/murex/testing/package/c4cf4727ec6abec83cfac5a4693c657b37ece27d/include/boost/smart_ptr/scoped_ptr.hpp:88:30:   required from ‘boost::scoped_ptr<T>::~scoped_ptr() [with T = cucumber::internal::UnixSocketServer]’
/home/matt/.conan/data/cucumber-cpp/0.4/murex/testing/build/232742180d6b48ffcaf9de5114f5a9b14f6df7e4/cucumber-cpp-0.4/tests/integration/WireServerTest.cpp:175:7:   required from here
/home/matt/.conan/data/boost/1.66.0/murex/testing/package/c4cf4727ec6abec83cfac5a4693c657b37ece27d/include/boost/core/checked_delete.hpp:34:5: error: deleting object of polymorphic class type ‘cucumber::internal::UnixSocketServer’ which has non-virtual destructor might cause undefined behaviour [-Werror=delete-non-virtual-dtor]

Environment:

Distributor ID:	Ubuntu
Description:	Ubuntu 16.04.3 LTS
Release:	16.04
Codename:	xenial
gcc (Ubuntu 5.4.1-2ubuntu1~16.04) 5.4.1 20160904
boost 1.66.0

Types of changes

• Bug fix (non-breaking change which fixes an issue).
• New feature (non-breaking change which adds functionality).
• Breaking change (fix or feature that would cause existing functionality to not work as expected).

Checklist:

• It is my own work, its copyright is implicitly assigned to the project and no substantial part of it has been copied from other sources (including Stack Overflow). In rare occasions this is acceptable, like in CMake modules where the original copyright information should be kept.
• I'm using the same code standards as the existing code (indentation, spacing, variable naming, ...).
• I've added tests for my code.
• I have verified whether my change requires changes to the documentation
• My change either requires no documentation change or I've updated the documentation accordingly.
• My branch has been rebased to master, keeping only relevant commits.

[matlo607]

@paoloambrosio, @muggenhor or @konserw : could you please review the pull request ?
Please ignore the commit 9827adc, konserw told me it's wrong. I will wait for #184.

[coveralls]

Coverage decreased (-0.02%) to 63.307% when pulling 176e003 on matlo607:missing-virtual-constructor into 70fa12c on cucumber:master.

[konserw]

Merged as part of #189