Close out the rest of oauth compatibility issues (#186)

* Update client to accommodate oauth2client>=4.0

This changes has been needed for a while now. The main blocker
seems to be the use of locked_file for caching GCE credentials.
I've added a simple multiprocess lockable file cache that uses
a similar approach to that used in ouath2client's multiprocess
file storage.

Submission of this should close issue #162.

* Update test runner setup
4 files changed