Add test for plain Ruby app with MRI 2.1.3

Heroku recently changed the base URL for prebuilt binaries from S3 (see https://github.com/heroku/heroku-buildpack-ruby/issues/304), so older versions of Ruby buildpack will fail when attempting to download the latest version of Ruby. I have added this test to verify both that Ruby buildpack is working and correct S3 URLs are being used.