changed version range of verror from ^ to ~

This fixes issues with node 0.8.x and 0.9.x (and possibly others) which could not install verror with ^1.4.0
Node 0.10.x installs verror without issues.

An example where this error occurs is when installing the dependencies of node-dns on Travis with Node 0.9.x: https://travis-ci.org/tjfontaine/node-dns/jobs/38713208#L69