WIP: CI: Don't install Recommends, etc.

• Don't install weak dependencies (Recommends). In particular this avoids installing the TeX toolchain, mitigating #242 (closed). However, because this also results in the OS's dbus package not being installed (!), it means we have to take responsibility for creating the messagebus user.
• Make adduser properly non-interactive. It's currently run < /dev/null as a workaround for its interactive prompts, but the --gecos option turns off the interactivity.
• Install yelp-tools, for yelp-build, so we can build doc/dbus-api-design.duck in recent enough distros.

https://travis-ci.org/smcv/dbus/builds/463852415