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/
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