fix(deps): require protobuf<4.0.0 on v1 branch (#460)

Author: parthea

setup.py

@@ -26,6 +26,7 @@
 dependencies = [
     "google-api-core[grpc] >= 1.31.5, <3.0.0dev,!=2.0.*,!=2.1.*,!=2.2.*,!=2.3.0",
     'enum34; python_version < "3.4"',
+    "protobuf<4.0.0dev",
 ]
 extras = {
     "pandas": "pandas>=0.17.1",