Bump php version

[Taluu]

Because 5.4 is now EOL for a while. Should bump to 5.6 (.. or 7 ?). As we are trying to stick close to [http://semver.org](Semantic Versionning), this should be considered on next major bump ("version 2.0")

[Taluu]

For the 1.5 (if it is ever released), I was thinking of bumping just to 5.5 or 5.6, and then for php 7 bumping to 2.0, as the public api may change a little.