build.sh: Correct test for rubygems
build.sh: Correct test for rubygems

Apparently, on some systems, rubygems *is* available when checked with apt-cache. Use dpkg -p to weed these out.

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