remove native-dns and use dig directly

native-dns module is unmaintained and we keep getting sporadic
errors from that module

Fixes #220

src/dns/digitalocean.js

@@ -12,7 +12,7 @@ var assert = require('assert'),
     async = require('async'),
     constants = require('../constants.js'),
     debug = require('debug')('box:dns/digitalocean'),
-    dns = require('native-dns'),
+    dns = require('dns'),
     SubdomainError = require('../subdomains.js').SubdomainError,
     superagent = require('superagent'),
     util = require('util');