chore(sauce): reduce browsers to only Safari

Chrome and iOS, iOS especially, take too long. They are buggy and
frequently cause the Travis worker boxes to crash.
Why don’t we run the unit tests solely on PhantomJS? PhantomJS doesn’t
have support for modern ES5 features like Object getters and
Function.prototype.bind that traceur depends on. Therefore, we have to
run tests in a modern browser.
Safari is the best choice because it is much faster than iOS (the
target platform), but also much more similar to it than Chrome (Chrome
uses v8, Safari/iOS uses Nitro).