Change serializers.Field to serializers.ReadOnlyField for 3.x compatibility

Using this code snippet as-is on DRF 3.0.1 results in a NotImplementedError
being raised for `to_representation` due to 3.0's changed behavior with
serializers.Field. The code snippet should be updated to use
serializers.ReadOnlyField instead.

Further discussion here:
https://groups.google.com/forum/?fromgroups=#!topic/django-rest-framework/aX8mLT_v3yc