replacing gzip with pigz to improve slugbuild time

pigz is a parallel implementation of gzip that takes advantage of multiple cpus and has reportedly improved slug building results as seen here:
http://www.paasmag.com/2014/02/11/git-push-heroku-master-now-40-faster/

Signed-off-by: Paulo Fagiani <pfagiani@gmail.com>
Merge pull request #10 from fagiani/pigz

replacing gzip with pigz to improve slugbuild time