A few weeks late to the party, I know, but I just tried using Geolocation and ran into this same problem.
The documentation has a few problems with it. First, it tells you to use the ionic/ionic module. However, it's not called ionic-framework/ionic.
Looking inside the ionic-framework/ionic.js, there is no reference to Geolocation.
Hopefully somebody else will see this bump and be able to point us in the right direction.