update engines.node in package.json

[zeke]

I'm using node 7 with yarn and got this error on installation:

error github-change-remote-file@4.2.0: The engine "node" is incompatible with this module. Expected version "6".
error Found incompatible module

This PR modifies the engines.node semver range to allow version 6 and greater.

[coveralls]

Coverage remained the same at 100.0% when pulling 5f55e51 on zeke:update-node-engine-range into 962486f on boennemann:master.

[christophwitzko]

Thank you!