(maint) Update dependencies to work with ruby1.9

Prior to this commit, mcollective-common could not install on Ubuntu
14.04. This is because Ubuntu 14.04 no longer provides packages for
rubygems. As of Ruby 1.9, rubygems is included in ruby, but many still
provide at least virtual packages for rubygems. This commit munges the
dependencies mcollective-common has to make sure it will install with
the correct dependencies on all systems.