Builds not requiring sudo access run in a container, which will have
better performance and less overhead on the Travis infrastructure.
Use the apt addon to install dependencies instead of explicit apt-get
commands.
Travis now offers Ubuntu Trusty (14.04 LTS) VMs as test runners, which
is gradually becoming the default. We can opt in to using Trusty now
so that we no longer need to manually update zlib to a newer version.
Download the test message database used for the T530-upgrade.sh test.
If the additional load on the web server is undesired, Travis can be
instructed to cache the file.
