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/route53.js

@@ -15,7 +15,7 @@ var assert = require('assert'),
     AWS = require('aws-sdk'),
     constants = require('../constants.js'),
     debug = require('debug')('box:dns/route53'),
-    dns = require('native-dns'),
+    dns = require('dns'),
     SubdomainError = require('../subdomains.js').SubdomainError,
     util = require('util'),
     _ = require('underscore');