manually reverted using mirror site. and use https protocols
use http, because http://ftp.ruby-lang.org is deliveried by fastly
rename hostname, ftp.ruby-lang.org is used by ftp protocol too.
Merge pull request #2198 from hsbt/revert-using-mirror

Manually reverted using mirror site. and use CDN serves.
allow easy defining multiple fallbacks for ruby, update #2198