Use protobuf 3.1 in travis.

.travis.yml

@@ -6,10 +6,24 @@ before_cache:
 cache:
   directories:
     - $HOME/.ccache
+    - $PROTOBUF_ROOT
+env:
+  global:
+    - USE_CCACHE=1
+    - PROTOBUF_ROOT=$HOME/protobuf
+before_install: |
+  if [[ $TRAVIS_OS_NAME == 'linux' ]]; then
+		# protoc
+    mkdir -p $PROTOBUF_ROOT
+		pushd $PROTOBUF_ROOT
+		wget https://github.com/google/protobuf/releases/download/v3.1.0/protoc-3.1.0-linux-x86_64.zip
+		unzip protoc-3.1.0-linux-x86_64.zip
+		popd
+		export PATH=$PROTOBUF_ROOT/bin:$PATH
+  fi
 install:
  - mkdir -p $HOME/.ccache
  - export PATH=/usr/lib/ccache:$PATH
- - sudo apt-get install protobuf-compiler
  - go get -u github.com/jteeuwen/go-bindata/...
  - go get -d github.com/ry/v8worker2
  - (cd $GOPATH/src/github.com/ry/v8worker2 && ./tools/build.py)