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).

**Example config**

```yaml
liip_monitor:
    checks:
        php_version:
            5.3.8: '>='
            5.3.16: '!='
            5.5.0: '<='
```

**Check result**

```bash
$ php app/console monitor:health
OK PHP Version ">=" "5.3.8": Current PHP version is 5.4.15
OK PHP Version "!=" "5.3.16": Current PHP version is 5.4.15
OK PHP Version "<=" "5.5.0": Current PHP version is 5.4.15
```