Add urllib3 AuthorizedHttp (#19)

diff --git a/tox.ini b/tox.ini
index 5f2df1c..ffaef94 100644
--- a/tox.ini
+++ b/tox.ini
@@ -9,6 +9,7 @@
   pytest-cov
   pytest-localserver
   urllib3
+  certifi
 commands =
   py.test --cov=google.auth --cov=google.oauth2 --cov=tests {posargs:tests}