Moves from removed 'rvm gem' command to 'gem'

the following error currently occurs when trying to use `rvm::define::gem`

    Notice: /Stage[main]/Gb::Ruby/Rvm::Define::Gem[bundler]/Exec[rvm-gem-install-bundler-ruby-2.0.0-p247]/returns:
    Please note that `rvm gem ...` was removed,
    try `gem install bundler --no-ri --no-rdoc` or `rvm all do gem install bundler --no-ri --no-rdoc` instead. ( see: 'rvm usage' )


switching from `rvm gem` to `gem` fixes the problem