- use CACHE_NVIM_DEPS_DIR
- do not cache pip
This is handled through http caches in general/better, and it is not
used much anyway.
- do not cache DEPS_DOWNLOAD_DIR
Built deps are cached, downloads are not needed then.
- display ccache stats before clearing
- do not cache ccache stats
- improve output of `du` (do not list pages of output for "/home/travis/.cache/go-build")