Make rbenv idempotent, install Ruby 2.1.4

* Don't repeat ourselves by using variables.
* Use more consistent variable naming and use of `local` in functions.
* Use `snake_case` for variable names and `ALLCAPS` for environment variables.
  https://github.com/thoughtbot/guides/tree/master/style#shell
* 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/