Allow using oauth2client versions < 2

Be compatible with older oauth2client versions. This simplifies
the installation (also for downstream) when software needs an older
oauth2client version but also wants to use googleapiclient.
diff --git a/setup.py b/setup.py
index d78f825..7dd6a0a 100644
--- a/setup.py
+++ b/setup.py
@@ -64,7 +64,7 @@
 
 install_requires = [
     'httplib2>=0.8,<1',
-    'oauth2client>=2.0.0,<3',
+    'oauth2client',
     'six>=1.6.1,<2',
     'uritemplate>=0.6,<1',
 ]