Add google-auth-httplib2 as dependency

As discussed here #2361 (comment)

This is used by https://github.com/google/google-api-python-client/blob/v1.6.6/googleapiclient/discovery.py#L371

Another option is to fix `luigi.contrib.bigquery` somehow.

tox.ini

@@ -24,6 +24,7 @@ deps=
   py27-gcloud: avro
   py33-gcloud,py34-gcloud,py35-gcloud,py36-gcloud: avro-python3
   gcloud: google-auth==1.4.1
+  gcloud: google-auth-httplib2==0.0.3
   google-compute-engine
   coverage>=4.1,<4.2
   codecov>=1.4.0