doc: change references to Stable to Current

With v6 we plan to rename the Stable release line to Current.
This commit updates all references to Stable in the codebase. It will
have to land along side updates to other repos within the org.

Refs: https://github.com/nodejs/nodejs.org/issues/669
PR-URL: https://github.com/nodejs/node/pull/6318


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Сковорода Никита Андреевич <chalkerx@gmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>