message: use docker image for ci with g++-4.8
diff: +image: "ubuntu:xenial"
  +
  +before_script:
  + - apt-get update -qq && apt-get install -qq -y make g++-4.8
  +
  build_linux:
  stage: build
  script:
  + - ln -s /usr/bin/g++-4.8 /usr/bin/g++
  + - g++ --version
  - make -j
  