Remove explicit requirement on rubygems.

It's better to not force the use of a package manager. Better to add it
to the global RUBYOPT if needed. Also, this fixes a dependency issue
when using the .deb package (rubygems1.9.1 is not required, and should
not be).
Merge pull request #288 from crohr/remove-rubygems-requirement

Remove explicit requirement on rubygems.