Change rubygems source

This solves deprecated message below:
$ bundle install
or
$ bundle exec rake spec:all

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.
All tests pass in ruby 2.0
Merge pull request #43 from sanemat/feature/rubygems-org

Replace rubygems source