There's a newer version of zstd, without any obvious changes that would
affect our build testing coverage.
Signed-off-by: David Sterba <dsterba@suse.com>
For a slight speed up of the build, cache reiserfs and zstd. A quick
cache validity is done, or it can be cleared manually on travis web UI.
Signed-off-by: David Sterba <dsterba@suse.com>