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

@@ -52,6 +52,7 @@
 
 REQUIREMENTS = [
     'google-api-core[grpc] >= 0.1.2, < 0.2.0dev',
+    'grpcio >= 1.8.2',
     'google-auth >= 1.0.2, < 2.0dev',
     'grpc-google-iam-v1 >= 0.11.1, < 0.12dev',
     'psutil >= 5.2.2, < 6.0dev',