This allows having multiple Dockerfile-base-* files without also needing to add a separate Dockerfile-travis-* file due to the FROM line changing.
(cherry-picked from commit 3f67c2fbf051aa9e322defb66c782477cd53b8bd)
This makes it simpler in the future to have branches that have dependencies that are different from master, rather than having a single image with all necessary dependencies installed, which may at some point end up not being possible to do.
(cherry-picked from commit 1bb5fdf2830515d33793c52f298d305bd12b5a60)
This replaces the custom python script which manipulated various files' mtime with ccache for allowing cmake builds to not need full recompiles every travis run.