Fixed missing dependency in tox file

Author: ekampf

tox.ini

@@ -8,6 +8,7 @@ deps =
     promise>=2.0
     six>=1.10.0
     pytest-mock
+    pytest-benchmark
 commands =
     py{27,33,34,py}: py.test graphql tests {posargs}
     py35: py.test graphql tests tests_py35 {posargs}