all: update to protoc v3.5.1 for testing

.travis.yml

@@ -8,7 +8,7 @@ go:
 
 install:
   - go get -v -d -t github.com/golang/protobuf/...
-  - curl -L https://github.com/google/protobuf/releases/download/v3.3.0/protoc-3.3.0-linux-x86_64.zip -o /tmp/protoc.zip
+  - curl -L https://github.com/google/protobuf/releases/download/v3.5.1/protoc-3.5.1-linux-x86_64.zip -o /tmp/protoc.zip
   - unzip /tmp/protoc.zip -d $HOME/protoc
 
 env: