Make Travis CI shut the fuck up about "source :rubygems"

The source :rubygems is deprecated because HTTP requests are insecure.
The source :rubygems is deprecated because HTTP requests are insecure.
The source :rubygems is deprecated because HTTP requests are insecure.
The source :rubygems is deprecated because HTTP requests are insecure.
The source :rubygems is deprecated because HTTP requests are insecure.
I GET IT!