build.sh: Check if rubygems is available on Ubuntu
build.sh: Check if rubygems is available on Ubuntu

On modern debian/Ubuntu versions, the contents of rubygems are included in ruby.
Do not try to install rubygems if the package is not there.

liqd/adhocracy,phihag/adhocracy,DanielNeugebauer/adhocracy,liqd/adhocracy,phihag/adhocracy,phihag/adhocracy,phihag/adhocracy,liqd/adhocracy,DanielNeugebauer/adhocracy,liqd/adhocracy,DanielNeugebauer/adhocracy,DanielNeugebauer/adhocracy,DanielNeugebauer/adhocracy,phihag/adhocracy