Enable PhpVersion Check

Added [ZendDiagnostics\Check\PhpVersion](https://github.com/zendframework/ZendDiagnostics#phpversion) to the Configuration.

There is also a small documentation fix [here](https://github.com/frne/LiipMonitorBundle/compare/liip:master...master#diff-04c6e90faac2675aa89e2176d2eec7d8L58).