Fix build: lock unzip dep

Some sort of regression in unzip 0.1.5 when they tried to make it node 0.10.x compatible. I think we can lock it for now, until its fixed.

See Also

* nearinfinity/node-unzip@d40cead09a8aa184c4bc5294f0665a0bcdf9f803
* nearinfinity/node-unzip#20

/cc @satazor