Update to kss 0.4.0

0.4.0 contains backwards compatible bugfixes, so the gulp plugin should update. I tried updating locally and everything worked fine.

Kss-node is actually on version 1.0.0 now, so updating all the way should be considered. But 1.0.0 requires Node 0.10.x or later, and I don't know if anybody considers that a problem??
Merge pull request #13 from davidtheclark/patch-2

Update to kss 0.4.0