remove unused require

src/mailer.js

@@ -37,7 +37,6 @@ var assert = require('assert'),
     async = require('async'),
     config = require('./config.js'),
     debug = require('debug')('box:mailer'),
-    dns = require('native-dns'),
     docker = require('./docker.js').connection,
     ejs = require('ejs'),
     nodemailer = require('nodemailer'),