gemfile: use HTTPS rubygems source

Use HTTPS for the connection to rubygems.org.

This fixes a warning in 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.
Merge pull request #71 from ktdreyer/https-rubygems

gemfile: use HTTPS rubygems source