Fix our CMake version dependency: 3.1

[muggenhor]

I.e. inside FindGMock.cmake we're using find_dependency() from the
CMakeFindDependencyMacro module. That module only got added in CMake
3.0. Furthermore we're relying on the import target Threads::Threads,
which only gets added with CMake 3.1.

So bump our requirement to what it is we really need.

[konserw]

What about #140 which does also that ?

[muggenhor]

That PR indeed also includes this change, but it is not about the same change. That's adding a featurea large change, this is just the small bug fix and thus smaller and quicker to get in. If you're worried about merge problems: this should be auto-mergable by git.

[muggenhor]

#140 looks almost good to go, so I'll close this one. I'll just make an issue report for it instead.