Be compatible with php 5.4

[kpn13]

Q	A
Bug?	no
New Feature?	no

Hi,

If I'm right, this lib is not compatible with PHP 5.4 just because of "::class" used in tests. Are you against a PR which will replace "::class" by the fully qualified class name ?

Best regards.

[kpn13]

Sorry I just saw #61 so I imagine be compatible with 5.4 is not an option anymore.

[Nyholm]

Thank you for this issue. I see no reason to do work to support legacy versions of PHP.

As soon as there is a PR with a decent reason to drop PHP 5.6 I would be happy to merge it =)