Set node version / npm version from package.json

[YayC]

It seems to be a best practice to specify node and npm versions within the package.json itself, under "engines" (see heroku docs on deploying node apps).

[gjaldon]

@YayConnolly thanks for opening this issue. Will look into supporting that with the next week.

[gjaldon]

Closing since this is in reference to the node buildpack from heroku. The current way of setting npm and node versions should be fine and the same with how heroku-buildpack-elixir does it