use forked ruby buildpack for multi test

I'm not saying we necessarily merge this but if we want this test to be run before the heroku guys merge the `export` fix, then this will work.

 - uses my forked ruby buildpack that merges https://github.com/heroku/heroku-buildpack-ruby/pull/319
 - upgrades compass (1.0.1) to be compatible with grunt-contrib-compass

closes #832