Fix `rbenv_gem` on Ruby 2.6+ (#15)
Fix `rbenv_gem` on Ruby 2.6+ (#15)

RubyGems shipped with Ruby 2.6 replaced the `--no-rdoc --no-ri` options
with `--no-document`. This PR allows the `RbenvRubygems` provider to use
the correct parameters based on the Ruby version it is currently running
on.
transportapi/rbenv-cookbook,transportapi/rbenv-cookbook,transportapi/rbenv-cookbook