Make AgencyChain.agencies a "resource collection" field

We can now get a `Query` when accessing the `agencies` attribute on an
`AgencyChain` instead of a dict with an URI to the agencies list.

gapipy/resources/booking/agency_chain.py

@@ -7,12 +7,12 @@
 
 class AgencyChain(Resource):
     _resource_name = 'agency_chains'
+    _is_parent_resource = True
 
     _as_is_fields = [
         'id',
         'href',
         'name',
-        'agencies',
         'agent_notifications',
         'communication_preferences',
         'flags',
@@ -28,3 +28,5 @@ class AgencyChain(Resource):
     _resource_fields = [
         ('booking_company', BookingCompany),
     ]
+
+    _resource_collection_fields = [('agencies', 'Agency')]