Use cached gems for plugins

Ok, this *seems* to work. We explicitly add the URLs to gems on rubygems to the packaging, and the build job downloads them and adds them to foreman's cache. The only niggle is shared deps (i.e deface) for which we now have to create new packages. On the plus side, id we ever convert to non-vendored, at least we'll have all the packages.

Supporting info on the testing is here: http://pastie.org/9509814

Some example builds on Jenkins:
column_view: http://ci.theforeman.org/job/packaging_build_deb_plugin/137/label=debian/console
deface: http://ci.theforeman.org/job/packaging_build_deb_plugin/136/label=debian/console
In theory yes, but since all the packages do "bundle update $PLUGIN_NAME" I figured I'd keep this in just to be safe.