Added a source line to the Gemfile

So bundle will default to fetching dependencies online if they're not installed locally.

This is what happens if you try to use `bundle install` without the dependencies installed on your local machine:

    [milesmatthias]$ bundle install
    Your Gemfile has no remote sources. If you need gems that are not already on your machine, add a line like this to your Gemfile:
        source 'https://rubygems.org'
    Could not find gem 'rest-client (>= 0) ruby' in the gems available on this machine.