Restrict changes to idempotency

* [Avoid captive user interfaces][1].
* [Don't be chatty][2].
* Move `exec $SHELL -l` line to spot recommended by [rbenv README][3].

[1]: http://goo.gl/cocdC
[2]: http://goo.gl/mL7hn
[3]: https://github.com/sstephenson/rbenv/