use ENV variables to configure vendor_url as sometimes we have an internal vendor mirror
Merge pull request #238 from yangchenyun/master

use ENV variables to configure vendor_url
Only reinstall ruby if the version has changed
Let RubyVersion build the buildpack version string
Just skip the downloading if there's a cached ruby
UI improvements
debug
Actually cache the fetched Ruby
Extract the Ruby fetching code to a method
Fetch when there's a cache-miss too
Typo
Use bundler 1.6.1