Just install ruby-install, don't call setup.sh

Hi, and thanks for `chruby` -- we're using it here at RailsBridge in our VM image to keep things simple. It's worked great.

I'm wondering if you would consider having the setup script *not* call ruby-install's setup script. We want to use the rest of the setup script to make sure everything is standard, but we only want to build one specific ruby, with a version constraint and build options, to keep build times down and the image that students have to download small.

ruby-install's setup just runs `make install` and then builds 3 different Rubies (with no options or way to pass versions/options in), so I think it's not too inconvenient for users to run `setup.sh` and then choose which Ruby they want to build. I've updated the README to explain this (there's already a section listing different Rubies you can build with ruby-install).

If you don't want to default to this, some way that I could optionally turn off the build step (by passing in an environment variable? I don't know... you could try to pass in information about which Rubies to build in argv but I think it would get complicated) would work for us. Thanks