Hide default php version

This PR adds a checkbox to hide the default PHP version from the selectable versions. The goal is to be able to hide the OS-default PHP in case you don't want to offer that to users. Eg CentOS 6 ships with PHP 5.4 and you might not want to allow users to pick that.

One way to solve that would be to just point the 'Default' to an additional PHP version, but that would implicitly upgrade everybody who has selected the 'default' version whenever that is changed, so that's bad for the stability of the user's websites.