Update kerl to use $_KERL_ACTIVE_DIR for get_active_path

* Define `$_KERL_ACTIVE_DIR` when activating
* Use `$_KERL_ACTIVE_DIR` for `kerl status`
* Unset `$_KERL_ACTIVE_DIR` when executing `kerl_deactivate`

(Using `$REBAR_PLT_DIR` was a bad idea and didn't work so I fixed it)