Made sure, the ruby sqlite3 package is installed when the default DB configuration is used.

Until now, one could run into a situation like this:

    Error: Could not retrieve catalog from remote server: Error 400 on SERVER: cannot load such file -- sqlite3

This PR fixes this by making sure, the Ruby sqlite3 package is installed when sqlite is used for the DB configuration (which is the default case).

As I don't have access to any RedHat box and the RedHat package index is only to RHN members, I can't tell for sure, what's the right package name on RedHat based distributions, so I left this line commented as TODO.

This is a proper/rebased followup to the messed up #52