fix for rbenv::gem always running

I've just added a -- to su. Without this, the --local option intended for gem list is instead applied to su, which results in a silent failure. Since the su returns non-zero, Puppet interprets this as needing to run rbenv::gem, which gets run during every Puppet apply regardless of whether the gem is already in place or not.
it seems this whole thing should be replaced with [Puppet::Util::Execution.execute](https://docs.puppetlabs.com/references/latest/developer/Puppet/Util/Execution.html#execute-class_method) - but for now, this should be fine.