Updated source declaration for rubygems (HTTPS)

Hey @jlindsey, here's another small one: I've updated the `source` declaration in the `Gemfile` in order to suppress the following warning issued by recent versions of `bundler`:

> The source :rubygems is deprecated because HTTP requests are insecure.
> Please change your source to 'https://rubygems.org' if possible,
> or 'http://rubygems.org' if not.

Thanks!