Update Python dependency to protobuf v3.5.0.post1
diff --git a/tools/dockerfile/test/cxx_ubuntu1404_x64/Dockerfile b/tools/dockerfile/test/cxx_ubuntu1404_x64/Dockerfile
index 6966d6b..990dac9 100644
--- a/tools/dockerfile/test/cxx_ubuntu1404_x64/Dockerfile
+++ b/tools/dockerfile/test/cxx_ubuntu1404_x64/Dockerfile
@@ -66,7 +66,7 @@
# 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