Requiring latest release of concurrent futures backport. (#4521)

See: agronholm/pythonfutures#63

Author: dhermes

setup.py

@@ -62,7 +62,7 @@
 ]
 
 EXTRAS_REQUIREMENTS = {
-    ':python_version<"3.2"': ['futures >= 3.0.0'],
+    ':python_version<"3.2"': ['futures >= 3.2.0'],
     'grpc': ['grpcio >= 1.7.0'],
 }