travis-ci: make Trusty builds a first class citizen
travis-ci: make Trusty builds a first class citizen

Trusty is used in Travis-CI, Windows Subsystem for Linux, and many other
places, so its an important target platform to test on.  This provides
testing for the backports in the Launchpad PPAs like:

* https://launchpad.net/~fdroid/+archive/ubuntu/fdroidserver
* https://launchpad.net/~fdroid/+archive/ubuntu/buildserver

f-droid/fdroid-server,f-droid/fdroid-server,f-droid/fdroidserver,f-droid/fdroid-server,f-droid/fdroidserver,f-droid/fdroidserver,f-droid/fdroidserver,f-droid/fdroid-server,f-droid/fdroid-server,fdroidtravis/fdroidserver,fdroidtravis/fdroidserver,f-droid/fdroidserver,fdroidtravis/fdroidserver,fdroidtravis/fdroidserver