Remove dependencies on httplib2 and google-auth-httplib2

This library still has dependencies on `httplib2` and `google-auth-httplib2`. Most of the other Google Python libs have [migrated away](https://github.com/googleapis/google-cloud-python/issues/1346) from these. Specifically, `httplib2` has [known thread-safety problems](https://github.com/googleapis/google-api-python-client/blob/master/docs/thread_safety.md), and `google-auth-httplib2` seems to have been developed as a backward compat package. Are there any plans for migrating these dependencies out?