Make rbenv idempotent, install Ruby 2.1.4

* Make better use of variables.
* Use more consistent variable naming and use of `local` in functions.
* Use Bash double-bracket.
* Set current shell to newest Ruby version.
* Don't install `--pre` version of Bundler.

https://www.ruby-lang.org/en/news/2014/10/27/ruby-2-1-4-released/
Line 12 was already defining `zshrc` as a local. You can do it ahead of time and don't need to do it at first assignment.