Use the secure S3 bucket for JRuby downloads

The JRuby team recently
[tweeted](https://twitter.com/jruby/status/525726366533443584) that Ruby
installers should switch to using a new secure link over HTTPS to
download JRuby artifacts. This patch switches the Ruby mirror to the new
URL!

Signed-off-by: David Celis <me@davidcel.is>