Decrease node requirement to =>0.6.0

I needed to use tar.gz as an easy drop in to create compressed files of a directory. None of the code in this lib, nor do the dependencies need 0.8.0. Since I was running 0.6.0, I needed  to downgrade the node engine requirement.

Consider making this change to those of us still using 0.6.x can use tar.gz. Thanks.