Remove blinker dependency

graphene/signals.py

@@ -1,4 +1,9 @@
+try:
     from blinker import Signal
+except ImportError:
+    class Signal(object):
+        def send(self, *args, **kwargs):
+            pass
 
 init_schema = Signal()
 class_prepared = Signal()

setup.py

@@ -55,7 +55,6 @@ setup(
 
     install_requires=[
         'six>=1.10.0',
-        'blinker',
         'graphql-core==0.4.9',
         'graphql-relay==0.3.3'
     ],