Use HTTPS RubyGems source as the :rubygems one is insecure & deprecated

Bundling with the `:rubygems` source prints a 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.

This fixes that and switches to HTTPS for security.