Only mention the lowest required PHP version in README

[svenluijten]

Checklist

• I have made a material change to the repo (functionality, testing, spelling, grammar)
• I have read the [Contribution Guide] and my PR follows them.
• I updated my branch with the master branch.
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation about the functionality in the appropriate .md file
• I have added in line documentation to the code I modified

Short description of what this PR does:

This PR makes sure the README mirrors what is actually required in the code. As per the composer.json, this library requires PHP 5.6 or higher. Let's not confuse users by only mentioning PHP 5.6 and 7.0 in the README.

[SendGridDX]

All committers have signed the CLA.

[codecov]

Codecov Report

Merging #106 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master    #106   +/-   ##
======================================
  Coverage    84.7%   84.7%           
======================================
  Files           3       3           
  Lines         170     170           
======================================
  Hits          144     144           
  Misses         26      26

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[thinkingserious]

Hello @svenluijten,

Thanks again for the PR!

It's HACKTOBERFEST! We want to show our appreciation by sending you some special Hacktoberfest swag. If you have not already, could you please fill out this form so we can send it to you? Thanks!

Team SendGrid DX

[thinkingserious]

Hello @svenluijten,

Thanks again for the PR!

We noticed that you have earned 1 out of the 5 points needed to receive glorious SendGrid Hacktoberfest swag.

Please take a moment to checkout this link to find more issues to get you past the required threshold.

Also, please be sure that you have officially registered here.

Thank you and Happy Hacktobering!

Team SendGrid DX

[thinkingserious]

Hello @svenluijten,

Thanks again for the PR!

You have earned 1 out of the 5 points needed to receive glorious SendGrid Hacktoberfest swag.

Please take a moment to checkout this link to find more issues to get you past the required threshold or to simply continue the celebration.

Also, please be sure you have officially registered with us here by November 1, 2018 to qualify.

If you have any questions you can email us at [email protected].

Thank you and Happy Hacktobering!

Team SendGrid