added possibibility to restrict PHP versions to a minimum version in the configBlock

Some PHP libraries might need to restrict the available PHP versions for new projects to a certain minimum version range (e.g. https://github.com/pulse00/Symfony-2-Eclipse-Plugin/commit/97d37b3ce18947d3f56be0899d3d35af9984737a). 

This PR adds the possibility to the `PHPVersionConfigurationBlock`.