whatwg-fetch, ember-fetch devDependency, and addon-docs upgrade

[ryanto]

ember-cli-fastboot-testing no longer has a dependency on ember-fetch, which solves the issue reported in #21. ember-fetch has now moved to a devDependency so we can run addon-docs in fastboot.

The reason this was doable is because we're only using fetch to pull fastboot instance data from ember-cli, outside of the lifecycle of the Ember application. Since our use case for fetch is so simple, we can get away with using whatwg-fetch directly.

Also, this PR upgrades addon docs 🎉

Related:

#21

ember-fastboot/ember-cli-fastboot#597

ember-cli/ember-fetch#98

ember-cli/ember-fetch#126