Stop warning on uninitialized variable errors
Add friendly error message for missing dependencies

* Major refactor of CF extensions / helpers
* Improved test coverage for CF extensions / helpers

[Finishes #80403676]
Support Heroku ruby binaries, 2.1.3 and above.

* Introduce code to check requested ruby version
* add /cedar/ to binary URLs for rubies over 2.1.3

[#80934254]
Only change URLs for MRI.

* Exclude JRuby
* Exclude ruby-build

[#80934254]