update README.md on ubuntu and gcc version

Summary: it seems since D15286181, we updated docker os_image to ubuntu18 and gcc7. this diff changes the README.md to reflect the change.

Reviewed By: yfeldblum

Differential Revision: D15513580

fbshipit-source-id: 0a3518c21c912903921946850bd08c948f09c295

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: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