update to work with fastboot 1.0

[mcfiredrill]

See the upgrade guide, basically the environment variable has been
removed.
https://gist.github.com/kratiahuja/d22de0fb1660cf0ef58f07a6bcbf1a1c#using-processenvember_cli_fastboot-to-run-import-in-fastboot-build

[davewasmer]

Thanks for this PR @mcfiredrill!

I'm not familiar with the nodeAssets option you are using in index.js - would you mind linking to the docs for it? I'd love to understand it a bit better before merging this in, just to make sure I can wrap my head around it.

[mcfiredrill]

@davewasmer Sorry for the delay, I'm not totally sure but I believe that comes from the ember-cli-node-assets package
https://github.com/dfreeman/ember-cli-node-assets#addons-with-fastboot