Drop php-5.x supports

[peter279k]

As title, I think it's time to drop the php-5.x version supports.

The php-5.x versions are too old to maintain and they're not maintained by official team anymore.

[peter279k]

@kevinlebrun, do you have any suggestion about this?

[kevinlebrun]

We could drop the 5.x versions support and release a major version of the library with the last few changes. I will try to do this in the next few days.