Use correct thoughtbot/homebrew-formulae repo
Install gems after configuring bundler for parallel installation.
Remove linux-prereq script by switching to Bash

Required changes:

* Change #!
* Move pre-req actions directly into linux
* Switch uses of $+command for which (zsh-ism)
* Don't source ~/.zshrc to pickup new exports
* Add setting of zsh as $SHELL
* Update README

Ancillary changes:

* Switch [ with [[ (best practice)
* Switch if-newline-then with if; then (style)
* Use $( ) and $(( )) (best practice)
* Switch wget with curl (consistency)