fix for $watch on paths; closes issue #453

fix for $watch on paths broken in https://github.com/tombatossals/angular-leaflet-directive/commit/4eef2c9eaea1406c42b367265795539e20b12c42; closes issue #453.
The problem was probably a copy-paste bug - `true` intended to be the third parameter of `$watch` was passed to the newly-created promise chain instead (as the useless second parameter to `then`).