update example to more modern os and gcc

Summary:
Openr is only doing travis ci for ubuntu 16.04 with gcc 5 so having
the example with the older flavors can be misleading for our community.

Reviewed By: saifhhasan

Differential Revision: D7345130

fbshipit-source-id: 684cc8e58a650050b6f63c2ec20aac1857476da7

build/fbcode_builder/README.md

@@ -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:14.04 \
-  gcc_version=4.9 \
+os_image=ubuntu:16.04 \
+  gcc_version=5 \
   make_parallelism=2 \
   travis_cache_dir=~/travis_ccache \
     ./travis_docker_build.sh &> build_at_$(date +'%Y%m%d_%H%M%S').log