node_modules are not installed on heroku
This is because you've defined bower in your devDependencies object in package.json. When you push to Heroku, it will not install things listed in devDependencies, so you need to have bower listed in your normal dependencies object =)
If you want to install devDependencies, add the following in the config vars in heroku.
key = NPM_CONFIG_PRODUCTION value = false
If you are using command line execute the following command
heroku config:set NPM_CONFIG_PRODUCTION=false
https://devcenter.heroku.com/articles/nodejs-support#devdependencies