TODO

* Fix tests for rbenv::gem
* Provide option for system level setup
* Make global ruby installation optional
* Provide Vagrant integration instruction.

* Allow multiple user installations. ✔
* Remove hardcoded paths etc. ✔
* Provide instruction on how to use it ✔
* Decouple ruby-build etc. ✔
* Testing, testing, testing ... ✔
* Remove deprecated stuff ✔