Remove unused keyring test dependency (#496)

diff --git a/tox.ini b/tox.ini
index 444213e..c5e0e5f 100644
--- a/tox.ini
+++ b/tox.ini
@@ -9,7 +9,6 @@
        oauth2client4: oauth2client>=4,<=5dev
        google-auth
        google-auth-httplib2
-       keyring
        mox
        pyopenssl
        pycrypto==2.6