Better error message when trying to build using npm@2

[cowboyd]

npm@2 is the default version of npm install all the way up to node v4, but it does not install dependencies in a flat tree; a behavior on which ember-d3-helpers has a hard dependency.

This means that when you do try and build using node <= 4.x, you get very cryptic errors as documented in #29

It would make it much nicer if you got a concise error message on ember install ember-d3-helpers that it will not work with the current version of npm and which versions it is known to be compatible with and instructions on where to start.