Fixes ENOTFOUND for uncached packages

The expected entry in the response is "url", not "repo" (compare to https://github.com/oliversalzburg/private-bower/blob/778b2fb51bf624345294507fbc27e79bbd477596/lib/server.js#L266)

Also see: https://github.com/bower/registry-client/blob/master/lib/lookup.js#L118

Fixes #15, #74