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>