Support ember-concurrency v4

[Windvis]

It's now a v2 addon, which requires a bit more work setting them up. Apps might still want to start using this new version so we should try to support it, without dropping support for version v3 since we don't want to create a new major release yet.

We should do some node version checking to conditionally do the required work based on the installed version.

Installation guides:

• http://ember-concurrency.com/docs/installation/