message: update README.md on ubuntu and gcc version
  Summary: it seems since we updated docker os_image to ubuntu18 and gcc7. this diff changes the README.md to reflect the change.
diff: @@ -14,8 +14,8 @@ host or virtual machine that can run a reasonably modern version of Docker:
  ``` sh
  ./make_docker_context.py --help # See available options for OS & compiler
  # Tiny wrapper that starts a Travis-like build with compile caching:
  -os_image=ubuntu:16.04 \
  - gcc_version=5 \
  +os_image=ubuntu:18.04 \
  + gcc_version=7 \
  make_parallelism=2 \
  travis_cache_dir=~/travis_ccache \
  ./travis_docker_build.sh &> build_at_$(date +'%Y%m%d_%H%M%S').log
  