Allow homebrew ruby
Allow homebrew ruby

IIRC, we uninstalled homebrew ruby b/c it caused gotchas with detecting
the correct ruby version. I *think* that’s moot now b/c our rbenv
initialization is better.

Uninstalling ruby is a gotcha now b/c the popular vim Formula depends
on it. If you installed vim recently with `brew install vim`, then ran
this script, it would break your vim installation:

    $ vim
    dyld: Library not loaded:
/usr/local/opt/ruby/lib/libruby.2.3.0.dylib
      Referenced from: /usr/local/bin/vim
      Reason: image not found
    Trace/BPT trap: 5

kickstarter/laptop