以下の通りにpackage.jsonとweb.jsを記述しherokuにデプロイしましたが、以下のエラーでデプロイできません。

エラー内容

remote: -----> Building dependencies
remote:        Pruning any extraneous modules
remote:        Installing node modules (package.json)
remote:
remote:        > app_name@0.0.1 postinstall /tmp/build_11023d05cefa4e84389aa595131ba028
remote:        > bower install
remote:
remote:        sh: 1: bower: not found
remote:
remote:        npm ERR! Linux 3.13.0-49-generic
remote:        npm ERR! argv "/tmp/build_11023d05cefa4e84389aa595131ba028/.heroku/node/bin/node" "/tmp/build_11023d05cefa4e84389aa595131ba028/.heroku/node/bin/npm" "install" "--unsafe-perm" "--userconfig" "/tmp/build_11023d05cefa4e84389aa595131ba028/.npmrc"
remote:        npm ERR! node v0.12.5
  1 {
remote:        npm ERR! npm  v2.11.2
remote:        npm ERR! file sh
remote:        npm ERR! code ELIFECYCLE
remote:        npm ERR! errno ENOENT
remote:        npm ERR! syscall spawn
remote:        npm ERR! app_name@0.0.1 postinstall: `bower install`
remote:        npm ERR! spawn ENOENT
remote:        npm ERR!
remote:        npm ERR! Failed at the app_name@0.0.1 postinstall script 'bower install'.
remote:        npm ERR! This is most likely a problem with the package,
remote:        npm ERR! not with npm itself.
remote:        npm ERR! Tell the author that this fails on your system:
remote:        npm ERR!     bower install
remote:        npm ERR! You can get their info via:
remote:        npm ERR!     npm owner ls app_name
remote:        npm ERR! There is likely additional logging output above.
remote:
remote:        npm ERR! Please include the following file with any support request:
remote:        npm ERR!     /tmp/build_11023d05cefa4e84389aa595131ba028/npm-debug.log
remote:
remote: -----> Build failed

package.json

  1 {
  2   "name": "app_name",
  3   "version": "0.0.1",
  4   "private": true,
  5   "devDependencies": {
  6     "http-server": "^0.8.0",
  7     "bower": "1.4.1",
  8     "express": "4.13.0"
  9   },
 10   "engine": {
 11     "node": "0.12.4",
 12     "npm": "2.10.1"
 13   },
 14   "scripts": {
 15     "postinstall": "bower install",
 16     "prestart": "npm install",
 17     "start": "http-server -a localhost -p 8000 -c-1"
 18   }
 19 }

web.js

  1 var express = require('express');
  2 var app = express();
  3 var port = Number(process.env.PORT || 8000);
  4
  5 app.use(express.static(__dirname + '/'));
  6 var server = app.listen(port, function(){console.log('Listening on port %d', server.address().port);});

エラーを見る限りbower install ができていないみたいですが、どこが間違っているのかわからない状況です。どなたかおわかりになるかたぎ教授ください。よろしくおねがいします。

補足:
ちなみにこのpackage.jsonはhttps://github.com/angular/angular-seedを参考にしました。