Setup Browserstack testing.

[rwjblue]

I believe we should be able to use ember-cli-browserstack (Ember recently did the same migration).

We need to test in:

• Edge
• IE11
• Safari
• FireFox
• Chrome

[rwjblue]

@kategengler - What do we need to set this up?

[kategengler]

ember-cli-browserstack + add the browsers to testem.js + browserstack env vars (username & access key) set. Would you like me to PR?

[rwjblue]

That would be great (if you have the time)!