Requiring 'grpcio >= 1.8.2'. (#4642)

This is due to a nasty spinlock bug [1] that has been partially
fixed [2] in `1.8.2`.

[1]: grpc/grpc#9688
[2]: grpc/grpc#13665

Author: dhermes

setup.py

@@ -63,7 +63,7 @@
 
 EXTRAS_REQUIREMENTS = {
     ':python_version<"3.2"': ['futures >= 3.2.0'],
-    'grpc': ['grpcio >= 1.7.0'],
+    'grpc': ['grpcio >= 1.8.2'],
 }
 
 setup(