Remove deprecated :rubygems source from Gemfile

This is triggering a security-related deprecation warning:

  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.
Merge pull request #111 from jamiecobbett/remove_deprecated_source_from_gemfile

Remove deprecated :rubygems source from Gemfile