Correct key for "env_vars" in sample_host file
Rbenv-var file should be named .rbenv-vars

Fixes #89

Signed-off-by: Jeroen van Baarsen <jeroenvanbaarsen@gmail.com>
Merge pull request #91 from intercity/jb-89-fix-rbenv-wrong-file-name

Rbenv-var file should be named .rbenv-vars
Clean up binary package after installing

* Removes the `tar.bz2` package file after extracting the Ruby version
  inside rbenv/versions.
* Use a multiline string for the Ruby installation command.
* Code style fixes.