[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
[ci skip] Changelog
Merge commit 'f41c75121ed4cd5006d6b2a47913681cda9091d5' into run-bower-develop