Update kerl

* Let get_active_path to use $REBAR_PLT_DIR for the current active installation path
Update kerl

* Define $_KERL_ACTIVE_DIR when activating
* Use $_KERL_ACTIVE_DIR for `kerl status`
Merge pull request #60 from jj1bdx/jj1bdx-active-path-fix

Update kerl to use $_KERL_ACTIVE_DIR for get_active_path