Add /usr/local/rubies to default search path.

This adds rubies in the `/usr/local/usr` directory to the RUBIES variable in the same manner that rubies in `/opt/rubies/` or `~/.rubies/` are added. This is done to make it friendlier to setups using Homebrew.

The guidelines state tests are needed, but I am unsure of how to approach testing this. I would appreciate any advice on where to get started.