Change RubyGems source to https URL to avoid Bundler 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 #4 from andyw8/secure-rubygems-host

Change RubyGems source to https URL to avoid Bundler warning