Replace Specs2 with uTest

[kenbot]

Specs2 doesn't have a great ScalaJS story at the moment. We are not using any fancy features with our tests at the moment, we just need asserts and ScalaCheck.

If we move our tests to uTest instead, we should be able to run them cross platform, so ScalaJS is tested too.

[kenbot]

@etorreborre suggests that a Scala.js-compatible version of Specs2 might be only a month or two away; we can hold off on this for a while.