Refactor code cookbook for Mac

Move vim out to a separate class because OSX ships with it already
installed. The other two packages would work fine from homebrew.
ack-grep is called ack in homebrew.
Allow use without sudo

This isn't necessary on Mac with homebrew. Just use the user and group
straight from the running process.
Don't install compilers on Mac

Assume that I've already jumped through the hoops to install GCC.
Noop curl installation on Mac.
Include rbenv on Mac

Just install one version of Ruby for now.