Removed npm version requirement

As cors is not using the npm api directly and npm v2.0.0 was just released, I think its better to drop the npm requirement.

Fixes `npm WARN engine cors@2.4.1: wanted: {"node":">=0.10.0","npm":"^1.4"} (current: {"node":"0.10.31","npm":"2.0.0"})` when installing cors with npm 2.0.0