fix(deps): require protobuf<4.0.0 on v1 branch (#567)

Author: parthea

setup.py

@@ -31,6 +31,7 @@
 dependencies = [
     "google-api-core[grpc] >= 1.31.5, <3.0.0dev,!=2.0.*,!=2.1.*,!=2.2.*,!=2.3.0",
     "google-cloud-core >= 1.1.0, < 3.0dev",
+    "protobuf<4.0.0dev",
 ]
 extras = {
 }