Client doesn't work with PHP < 5.6.0

[hishamyounis]

The documentation specify that the client works with PHP >= 5.4.0. But classes ScopeValidator and TokenFetcher define array constants, which are only supported in PHP >= 5.6.0.

This throws the following error: Arrays are not allowed in class constants.

[ido567]

Hey @hishamyounis, thank you for catching that, I'm afraid I won't be able to attend to this in the next few days, but I would love you to issue a PR to fix that and I'll merge that.