#31 Use Node.js 0.10.26

From the [Heroku docs](https://devcenter.heroku.com/articles/getting-started-with-nodejs):

> Lastly, you should specify the version(s) of node that are compatible with your application by adding an engines field to your package.json. If you choose not to set this value, the default stable version of node will always be used when you deploy.

Parsing `package.json` is a bit outside the scope of this project, so I opted to just let it default.
We can see [here](https://github.com/heroku/heroku-buildpack-nodejs/blob/master/bin/compile#L24) how the Heroku Node.js buildpack uses semver.io to resolve which version to install, and [with a blank version range](https://semver.io/node/resolve?range=) you currently get `0.10.26`, so that's what I opted to use here.

I [built from scratch](https://github.com/ejholmes/vagrant-heroku#building-from-scratch) and it seems to work just fine!