Bump deps, drop node 6

[xg-wang]

No description provided.

[rwjblue]

Seems good to me, do we need to be worried about multiple major versions in play within the app at once anymore?

[xg-wang]

@rwjblue I'm hoping to get #292 in, then any fastboot app should be safe.

For multiple major in browser only app it's no different than other addons. Unfortunately there's no perfect solution now.

The best way for app to guard is to install https://github.com/salsify/ember-cli-dependency-lint.

For addon authors if the addon depends on a specific version ember-fetch (or any other addon) functionality, there's always risk if not enforce a peer-dependency.