fix(grunt): run express:prod on production port

closes #1814

[skip ci]

Author: Awk34

templates/app/Gruntfile(grunt).js

@@ -46,6 +46,7 @@ module.exports = function(grunt) {
       },
       prod: {
         options: {
+          port: process.env.PORT || <%= prodPort %>
           script: '<%%= yeoman.dist %>/<%%= yeoman.server %>'
         }
       }