Update Python dependency to protobuf v3.5.0.post1
diff --git a/requirements.txt b/requirements.txt
index 1296995..c976cef 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -3,6 +3,6 @@
 cython>=0.23
 enum34>=1.0.4
 futures>=2.2.0
-protobuf>=3.2.0
+protobuf>=3.5.0.post1
 six>=1.10
 wheel>=0.29