Fix Gemfile source

When running `bundle install` this warning shows up:

> 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.

This PR eliminates this warning by using `https://rubygems.org` instead of `:rubygems` as Gemfile source.