Use current node version for new applications.

If `node` is already available (by using multi-buildpacks or otherwise
manually vendoring a custom version) it will not install another node
version.

Otherwise, it will lock the version of node for applications as follows:

* Existing Applications - `0.4.7`
* New Applications - Will be locked to the latest node version supported
  by the `heroku-buildpack-nodejs` (using the `nodejs.manifest`
  published by that project) at the time of first publishing.