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/
Fix typo

https://github.com/thoughtbot/laptop/issues/319
Add $HOME/.rbenv to $PATH even if it's installed
Remove rbenv-gem-rehash plugin installation

This plugin was merged into rbenv
https://github.com/sstephenson/rbenv/pull/638
Remove redundant Foreman

It is installed earlier via https://toolbelt.heroku.com/
Remove Heroku Config

I haven't been using `heroku config:pull`, etc. for some time.
Instead, I've been using more targeted shell lines insides our
[`bin/setup`] convention, such as:

    if ! grep -F "FILEPICKER" .env > /dev/null; then
      heroku config --remote staging | grep "FILEPICKER" >> .env
      sed -i '' 's/\:/=/' .env
      sed -i '' 's/ //g' .env
    fi

[`bin/setup`]: http://robots.thoughtbot.com/bin-setup

This is more targeted and automated than using `heroku config:pull`.
Use Homebrew for rbenv and ruby-build

* Many thoughtbot employees are successfully using Homebrew'd rbenv.
* rbenv's recommended install is Homebrew for OS X:
  https://github.com/rbenv/rbenv#installation
* We use Homebrew / package management whenever possible for
  system dependencies.
* We save a step adding rbenv to `PATH` in zshrc / zshenv,
  because Homebrew does it for us.
  This is nice because we don't add `rbenv` to `PATH` in
  thoughtbot/dotfiles.
Use ShellCheck to enforce shell style

* Document in "Contributing" section.
* Use GitHub code fences to improve syntax highlighting.
* Don't be so Rails-y in the documentation.
Suggested installation includes an explicit review step

While we did previously state "Read, then...", that's easier to ignore.
This brings the copy-pastable commands in line with the intent: to have
the user review the script before executing it. This also removes the
potentially confusing use of process substitution.

Note: The presence of `less` is assumed to avoid assumptions about
editors or the noise of handling that robustly (e.g. `${EDITOR:-vim}`)
merge latest thoughtbot
merge readme