Drop PHP 5.x/7.0 support

[jakzal]

Both PHPUnit and PhpSpec regularly drop support for legacy PHP versions. Is it time for prophecy to do the same?

I suggest dropping any unmaintained PHP version and adopt a practice of doing it regularly.
Benefits of supporting less platforms would include less maintenance or being able to use more recent language features.

I'd be happy to help.

[Taluu]

I'd also be happy to help. I'm 100% for that, the 👍 was not enough to show my support. :D

[DonCallisto]

We had this #419 that, I suppose, could be closed once this issue will be closed.
BTW I'm in favor of this.

[Nek-]

This should be closed considering #644