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.

Closes https://github.com/heroku/heroku-buildpack-ruby/issues/140.