Add Embroider to CI tests

[reidab]

Adds test scenarios for the new Embroider build system to ember-try and to the Travis config.

These test currently fail since the generated pod-styles manifest can't be found. I've marked the failures as allowed in Travis.

[webark]

@reidab Thanks. I haven't been doing ember (or general front end for that matter) development for a bit, so sorry that I have left this project in somewhat limbo. I'd like to explore getting this version of ECC running on embroider (have you tried the beta release?) so I'd prefer to merge it onto a new "embroider" branch where we can work towards getting that working. Does that seem fair?