Error after deployment on heroku: Error R10 (Boot timeout) -> Web process failed to bind to $PORT within 60 seconds of launch.

server.js

var express = require('express'),
  stylus = require('stylus'),
  logger = require('morgan'),
  bodyParser = require('body-parser'),
  mongoose = require('mongoose');

var env = process.env.NODE_ENV = process.env.NODE_ENV || 'development';

var app = express();

function compile(str, path) {
  return stylus(str).set('filename', path);
}

app.set('views', __dirname + '/server/views');
app.set('view engine', 'jade');
app.use(logger('dev'));
app.use(bodyParser.urlencoded({extended:true}));
app.use(bodyParser.json());
app.use(stylus.middleware(
  {
    src: __dirname + '/public',
    compile: compile
  }
));
app.use(express.static(__dirname + '/public'));
/*
for switching between dev and prod environments.
 */
if(env === 'development'){
    mongoose.connect('mongodb://localhost/multivision');
} else {
    mongoose.connect('mongodb://shapan:shapan1234@ds143449.mlab.com:43449/multivision');
}

var db = mongoose.connection;
db.on('error', console.error.bind(console, 'connection error...'));
db.once('open', function callback() {
  console.log('multivision db opened');
});
var messageSchema = mongoose.Schema({message: String});
var Message = mongoose.model('Message', messageSchema);
var mongoMessage;
Message.findOne().exec(function(err, messageDoc) {
  mongoMessage = messageDoc.message;
});

app.get('/partials/:partialPath', function(req, res) {
    res.render('partials/' + req.params.partialPath);
});

app.get('*', function(req, res) {
  res.render('index', {
    mongoMessage: mongoMessage
  });
});

app.listen(process.env.port || 5000);

var port = process.env.port || 5000;
console.log('Listening on port ' + port + '...');