Update source line in Gemfile.

Fix the following bundler 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 #59 from CodeGnome/pull-reqs/fix_deprecated_rubygems_source

Update source line in Gemfile to source 'https://rubygems.org'