We update our base image to Ubuntu 18.04, and update all of our
dependencies to the latest released versions. This includes:
- gcc-multilib-arm-linux-gnueabihf (latest distro version)
- mdbook 0.2.1 (0.2.3 is out, but there's no binary build)
- OpenSSL 1.1.1b
- libpq 11.2