Drop oauth2client dependency  (#499)

diff --git a/setup.py b/setup.py
index 4796840..07a4440 100644
--- a/setup.py
+++ b/setup.py
@@ -64,7 +64,8 @@
 
 install_requires = [
     'httplib2>=0.9.2,<1dev',
-    'oauth2client>=1.5.0,<5.0.0dev',
+    'google-auth>=1.4.1',
+    'google-auth-httplib2>=0.0.3',
     'six>=1.6.1,<2dev',
     'uritemplate>=3.0.0,<4dev',
 ]