It's now available and allows us to have a better default environment,
with GCC 7.4.0 and Clang 7.
We now need GCC 7+ for C++17 support so it's more efficient to upgrade
the image than to install it on Ubuntu 16.04 (xenial).
Also fixes a couple -Wdeprecated-declarations warnings on macOS now
that we build against macOS 10.12.