Revert "Allow installation of bundler even when puppet is within rbenv"

This fix was not necessary because `rbenv exec` [prepends an entry to the path](https://github.com/sstephenson/rbenv/blob/master/libexec/rbenv-exec#L45) that will take precedence over the entry that's already there.

Additionally, passing `PATH` to an `exec` resource like this triggers puppet warnings, which leads to lots of noise in puppet runs.

This reverts commit 5a542313e956a48ef708cae7ae54d005a0fb66a1 (PR #8).  As mentioned in that PR, we've been using a version prior to this fix up until now without issue, so I'm confident that this is safe to revert.