Bump promise to 2.3 (#265)

It contains a fix for a thread-safety issue in Dataloader.

Author: jnak

setup.py

@@ -14,7 +14,7 @@
 requirements = [
     # To keep things simple, we only support newer versions of Graphene
     "graphene>=2.1.3,<3",
-    "promise>=2.1",
+    "promise>=2.3",
     # Tests fail with 1.0.19
     "SQLAlchemy>=1.2,<2",
     "six>=1.10.0,<2",