Update to ember-cli 2.13

[Dhaulagiri]

Built on top of https://github.com/poteto/ember-cli-flash/pull/233

• Swap out jshint for eslint
• Adds 2.8 lts to testing matrix (we should probably trim this entire list down in a separate PR)

[Dhaulagiri]

I'll let #236 land and then bring back some of the smaller pieces of this PR afteward