change usage of CHRUBY_SOURCE to CHRUBY_ROOT

Before, `CHRUBY_SOURCE` would point directly to the `chruby.sh` file. This has
now been changed to be more inline with other implementations[1].

`CHRUBY_ROOT` now points to `/usr/local` by default, and the path is always
expanded with `share/chruby/{chruby,auto}.sh`.

[1]: https://github.com/dgoodlad/puppet-chruby/blob/61f2db9ec7c70b4726855a9c615ce137713a2a1e/files/chruby.sh#L4