Merge pull request #292 from hasegawa-jun/fix-reference-error

make sure to call require()

lib/appenders/smtp.js

@@ -2,6 +2,7 @@
 var layouts = require("../layouts")
 , mailer = require("nodemailer")
 , os = require('os')
+, async = require('async')
 , unsentCount = 0
 , shutdownTimeout;