Add google-auth-httplib2 as dependency
Add google-auth-httplib2 as dependency

As discussed here https://github.com/spotify/luigi/pull/2361#issuecomment-378661375

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.
samuell/luigi,adaitche/luigi,PeteW/luigi,Magnetic/luigi,foursquare/luigi,rayrrr/luigi,soxofaan/luigi,dlstadther/luigi,Magnetic/luigi,riga/luigi,samuell/luigi,spotify/luigi,dlstadther/luigi,jamesmcm/luigi,Magnetic/luigi,samuell/luigi,rayrrr/luigi,foursquare/luigi,riga/luigi,soxofaan/luigi,samuell/luigi,spotify/luigi,PeteW/luigi,dlstadther/luigi,spotify/luigi,riga/luigi,PeteW/luigi,riga/luigi,dlstadther/luigi,rayrrr/luigi,Magnetic/luigi,spotify/luigi,soxofaan/luigi,foursquare/luigi,adaitche/luigi,soxofaan/luigi,jamesmcm/luigi,jamesmcm/luigi,rayrrr/luigi,PeteW/luigi,foursquare/luigi,adaitche/luigi,adaitche/luigi,jamesmcm/luigi