Add google.auth._oauth2client - helpers for oauth2client migration (#70)

diff --git a/tox.ini b/tox.ini
index c77487c..ad760bd 100644
--- a/tox.ini
+++ b/tox.ini
@@ -11,6 +11,7 @@
   urllib3
   certifi
   requests
+  oauth2client
   grpcio; platform_python_implementation != 'PyPy'
 commands =
   py.test --cov=google.auth --cov=google.oauth2 --cov=tests {posargs:tests}