fixed bundler source due deprecation warnings:

>> 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 #13 from loybert/fix/secure_bundler_source

Fixed bundler deprecation warnings, because "source :rubygems" is deprecated