Make sure latest version works on both PHP 5.6 and 7

[ademarco]

This is a test PR to assess if PHP 7 is actually a required dependency or if the project can work on PHP 5.6 too.

[ademarco]

@lstrojny this PR fixes tests allowing the latest version of the library to run on both PHP 7 and 5.6.

[lstrojny]

I would prefer not to go back to PHPUnit 5 but instead have a test suite that works with both 5 and 6, which should be relatively trivial. That way we can have both: 5.6 compatibility on the one hand but support for PHPUnit 6 as well.

[ademarco]

The problem is that PHPUnit 6 requires PHP 7, so we don't have much choice if we want to support both 5.6 and 7 at the same time than to rollback to PHPUnit 5.

[lstrojny]

@ademarco what about requiring "5|6" and having a tiny compatibility layer?

[ademarco]

Yep, that can be interesting, I'll try that!

[ademarco]

There we go! \cc @lstrojny

[ademarco]

@lstrojny Ok tests are back and now we use phpunit 6 on php 7 and phpunit 5.7 on php 5.6.

[lstrojny]

Thank you!

[ademarco]

Great, thanks!