Mock geocoding network calls.

I've been burned enough times by the `GeocoderTimedOut` errors, which cause us to restart the Travis builds This makes each Travis run to take at least 15 minutes longer. See #143.

Use the `mock` library to mock geopy's geocoding calls for our round-trip tests. The mocked methods are the `geocode` and `reverse` methods of one of the geocoders, which is GeoPandas interface to geopy.