Pin current Graphene-Django version to Graphene~2

To avoid compatibility issues it's better to pin dependencies.

Author: syrusakbary

setup.py

@@ -57,7 +57,7 @@
 
     install_requires=[
         'six>=1.10.0',
-        'graphene>=2.0',
+        'graphene>=2.0,<3',
         'Django>=1.8.0',
         'iso8601',
         'singledispatch>=3.4.0.3',