default lang is a string, not an array of strings. Fixes unit tests for last revert

lib/configuration.js

@@ -136,7 +136,7 @@ var conf = module.exports = convict({
     port: 'integer{1,65535}?',
     host: 'string?'
   },
-  default_lang: 'array { string }* = [ "en-US" ]',
+  default_lang: 'string = "en-US"',
   supported_languages: 'array { string }* = [ "en-US" ]',
   locale_directory: 'string = "locale"',
   express_log_format: 'string [ "default", "dev" ] = "default"',