Set min version of graphql-core to v3.1.1 (#1215)

Author: jkimbo

setup.py

@@ -82,7 +82,7 @@ def run_tests(self):
     keywords="api graphql protocol rest relay graphene",
     packages=find_packages(exclude=["examples*"]),
     install_requires=[
-        "graphql-core>=3.1.0b1,<4",
+        "graphql-core>=3.1.1,<4",
         "graphql-relay>=3.0,<4",
         "aniso8601>=8,<9",
     ],