FMG and CSS Xtras conflict -- allow ruby 1.8 to remain installed

When trying to install both FFmpeg support with Compass Tools, there is a package dependency conflict as FFmpeg support installs ```flvtool2``` which depends on ```ruby1.8```, but Compass Tools removes ```ruby1.8-dev libruby1.8 ruby1.8```.  

Because we're now using RVM to manage ruby, this won't cause any conflicts.