Revert to upstream angular-leaflet-directive plugin

Requires > v0.7.8 of the angular-leaflet-directive for
utfgrid support.

The changes we made to our internal fork were merged to
upstream and the version was updated.

Those with an existing install may need to manually run:
```
cd js/angular
bower update angular-leaflet-directive
```
in order to pull in the new dependency.
Merge pull request #151 from WorldBank-Transport/feature/update-angular-leaflet-dependency

Revert to upstream angular-leaflet-directive plugin