Update Python dependency to protobuf v3.5.0.post1
diff --git a/tools/dockerfile/interoptest/grpc_interop_csharpcoreclr/Dockerfile b/tools/dockerfile/interoptest/grpc_interop_csharpcoreclr/Dockerfile
index ea82476..af2ab90 100644
--- a/tools/dockerfile/interoptest/grpc_interop_csharpcoreclr/Dockerfile
+++ b/tools/dockerfile/interoptest/grpc_interop_csharpcoreclr/Dockerfile
@@ -62,7 +62,7 @@
 # Install Python packages from PyPI
 RUN pip install --upgrade pip==9.0.1
 RUN pip install virtualenv
-RUN pip install futures==2.2.0 enum34==1.0.4 protobuf==3.2.0 six==1.10.0 twisted==17.5.0
+RUN pip install futures==2.2.0 enum34==1.0.4 protobuf==3.5.0.post1 six==1.10.0 twisted==17.5.0
 
 #================
 # C# dependencies