Adding a usage message to install.rb script

Previously, invoking the install.rb script with zero arguments
caused it to attempt to install all gems without a version
suffix, which would fail spectacularly. Failing gracefully with
a usage message is more helpful to a first-time user.

This was a small pain point for me as I attempted to build
master for the first time on my local machine. Hope this helps
out other first-time contributors.