Use cache.ruby-lang.org for Ruby downloads

Ruby-lang.org uses a CDN for source downloads at cache.ruby-lang.org [0]
which seems to be more responsive than ftp.ruby-lang.org.

[0]: https://www.ruby-lang.org/en/news/2013/09/07/we-use-fastly-cdn/
Merge pull request #124 from jacobvosmaer/ruby_cache

Use cache.ruby-lang.org for Ruby downloads