Heroku does not read node version

Don't give 'x' on the version

 "node": "0.12.x",
 "npm": "2.5.x"

write complete version. Ex.

 "node": "0.12.0",
 "npm": "2.5.0"

This works for me -- make sure you've actually committed these changes to Git, and pushed the repository to Heroku. You may also want to specify exact Node and NPM release numbers for your Heroku app.

While this WILL WORK with the variable releases you've got specified, it's not recommended, as small changes in releases might cause issues for you.

For reference, here are the Heroku docs on specifying a Node.js runtime as well: https://devcenter.heroku.com/articles/nodejs-support#node-js-runtimes

Maybe your master branch is not the branch is not updated, try merging the branch that you want to deploy into master in order to use:

git push heroku master

@rdegges was right that the package.json wasn't correctly committed to Heroku. So just following the Heroku instructions didn't work for me for some reason. Here is what I had to do in order to make it work.

git clone <my git project>
heroku create <app name>

#remove package.json
mv package.json tmp_package.json
git add package.json
git commit -m "removed package.json"

#re-add package.json
mv tmp_package.json package.json
git add package.json
git commit -m "re-added package.json"

git push heroku master