Optimize build for easy and fast building on Travis

The current build artefacts are optimized for compatibility with a wide area of systems, in order to prevent users of AppImageKit from having to build from source. With more and more users using our tools as part of a contiuous build chain on Travis CI, some projects ask to build everything from source.

For example, users like Microsoft and Audacity may not be happy to pull precompiled code from a third party such as us, even though all of that code is open source and built on Travis CI in the open.

We could offer, in addition to what we have now, a second build path that is optimized for easy and fast building on Travis CI. I think this would look greatly different from what we currently have, since we would probably need no Docker, need to do much less building of common third-party parts, and would not have to do as much semi-static linking and testing. The objective of this second build path which should cover both `appimagetool` and `linuxdeployqt` would be to get a functional `appimagetool` and `linuxdeployqt` compiled on Travis CI in the shortest amount of time possible, while having to run nowhere else but on that very instance of Travis CI. I think it would also be fine that the runtime, when built in this way, would not be compatible with older systems, since the content of the AppImage built this way by the AppImageKit user would equally not run on older systems. As an extra measure, we could even put in a check so that `appimagetool` and `linuxdeployqt` built this way would refuse to run on any other system.