Bump min PHP version to 5.5?

[Konzertheld]

Hello people,

I just fixed our requirements test to reflect the change that we require PHP 5.4 as of Habari 0.9.2.

I suggest that for Habari 0.10, we require at least PHP 5.5. My reasons:

* The support cycle for PHP 5.5 has ended, so everyone is encouraged to upgrade to 5.6 anyway.

* Active support for 5.6 is going to end soon.

* PHP 5.5 is included in Ubuntu 14.04 (that's two and a half years old).

Christian