In case anyone else comes here, after trying the fixes here my app still had errors launching on Heroku, but the error messages were not verbose and only continued to give the Node version error. This series of fixes got it going.
1. in terminal, npm install - for me this re-installed npm. I also ran "meteor update" but I knew there was no significant update that would break my app.
2. check the .gitignore file - something important might be in there. In my case, "newrelic" folder was in there but not referenced as a dependency in package.json. To fix this make sure all subfolders have a dependency reference in package.json (recommended) or else remove node_modules from .gitignore (creates extra fetching, not recommended but works).
3. use Kevin's build path above - or another updated Meteor build package - I used the horse package. Make sure you add .git to the end of the URL like so:
heroku buildpacks:set https://github.com/kevinseguin/heroku-buildpack-meteor.git --app myAppName
4. set the $PATH - in terminal:
PATH=$PATH:$HOME/.meteor
5. set the ROOT_URL in Heroku - some instructions out there say to set it as "myAppName.herokuapp.com". This causes an error in Heroku. It is fixed if you prepend, "http://", like so, "http://myAppName.herokuapp.com".