Keep caches in home directory instead of project
Keep caches in home directory instead of project

This was a workaround purely for dev workstations, and is actually not
worth the effort. Mixing Docker build and local-environment build on a
dev workstation breaks 'pub' - because it saves links to package
directories in project-specific .packages file, and the links are
different between Docker and local builds.

Directories will be mounted via 'docker run --mount'.

dasfoo/travis