travis/prepare: use lz4 compression type for pkgs/repos.
travis/prepare: use lz4 compression type for pkgs/repos.

lz4 is the fastest one available with libarchive, and
size is not the most important thing in travis.

necrophcodr/void-packages,necrophcodr/void-packages,necrophcodr/void-packages,necrophcodr/void-packages,necrophcodr/void-packages