[fix #171] Refactor Rake Detect and Run

Move rake task detection and running to a separate independently tested class.

Compile exits when rake tasks cannot be successfully detected with error message
Merge pull request #172 from heroku/schneems/better-rake-detect

[fix #171] Refactor Rake Detect and Run
Merge branch 'master' of github.com:heroku/heroku-buildpack-ruby