Make doctests more resilient to API data changes

[perrygeo]

This doctest is suddenly failing, most likely due to changes in the geocoder data.

=================================== FAILURES ===================================
_________________________ [doctest] docs/geocoding.md __________________________
150 ## Reverse geocoding
151 
152 Places at a longitude, latitude point may be found using `Geocoder.reverse()`.
153 
154 ```python
155 
156 >>> response = geocoder.reverse(lon=-73.989, lat=40.733)
157 >>> response.status_code
158 200
159 >>> for f in response.geojson()['features']:
Differences (unified diff with -expected +actual):
    @@ -1,5 +1,5 @@
     Atlas Installation, 124 E 13th St, New York, New York 10003, United States: poi.2701346205
     Gramercy-Flatiron, New York, 10003, New York, United States: neighborhood.21161
    -New York, New York, United States: place.37501
    +New York, New York, United States: place.34530
     10003, New York, United States: postcode.2254639497
     New York, United States: region.628083222
/home/travis/build/mapbox/mapbox-sdk-py/docs/geocoding.md:159: DocTestFailure
===================== 1 failed, 1 skipped in 0.19 seconds ======================

Fixing manually for now but we should anticipate this happening fairly regularly and make the tests more resilient.