[Testing] add multi stage build testing

[gabrielcsapo]

Would it be possible to add a multi stage matrix to the travis script that installs broccoli and ember-cli, then links the packages in the repo and runs broccoli and ember-cli's tests to ensure compatibility?

[lynchbomb]

I've not done that before. Let me take a look at the Travis API. Obviously we will keep this applied specifically to the v4 branch