Pin oauth2client to 1.5.2
Pin oauth2client to 1.5.2

This addresses https://github.com/GoogleCloudPlatform/gsutil/issues/330
but only when gsutil 4.17 is released and pushed to PyPi.

GoogleCloudPlatform/gsutil,fishjord/gsutil,BrandonY/gsutil,GoogleCloudPlatform/gsutil