Install protobuf on CI

.travis.yml

@@ -1,5 +1,10 @@
 language: rust
-cache: cargo # https://docs.travis-ci.com/user/caching/#Rust-Cargo-cache
+
+cache:
+  directories:
+    - $HOME/.cargo
+    - $HOME/protobuf
+    - $TRAVIS_BUILD_DIR/target
 
 rust:
   - stable
@@ -54,3 +59,8 @@ script:
   - cargo build --verbose --all-features
   - cargo test --verbose --all-features
   - cargo doc --verbose --all-features --no-deps
+
+before_install:
+  - export PATH=$PATH:$HOME/protobuf/bin
+  - export PROTOC_VERSION=$(cat PROTOC_VERSION)
+  - bash install_protobuf.sh

PROTOC_VERSION

@@ -0,0 +1 @@
+3.5.1

install_protobuf.sh

@@ -0,0 +1,18 @@
+#!/usr/bin/env bash
+set -e
+
+PROTOC_VERSION=$(cat PROTOC_VERSION)
+
+check_protoc_version () {
+    this_version=`protoc --version`
+    return `[ "libprotoc $PROTOC_VERSION" = "$this_version" ]`
+}
+
+if check_protoc_version; then
+    echo $PROTOC_VERSION detected.
+    exit
+fi
+
+wget https://github.com/google/protobuf/archive/v$PROTOC_VERSION.tar.gz
+tar -xzvf v$PROTOC_VERSION.tar.gz
+cd protobuf-$PROTOC_VERSION && ./autogen.sh && ./configure --prefix=$HOME/protobuf && make && make install