Change Gemfile source to https://rubygems.org to stop 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 #54 from mmb/rubygems_source

Change Gemfile source to https://rubygems.org to stop warning.