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]: https://github.com/grpc/grpc/issues/9688
[2]: https://github.com/grpc/grpc/pull/13665
diff --git a/setup.py b/setup.py
index ede41b1..8c8ea18 100644
--- a/setup.py
+++ b/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(