Assertion output regression

In master (https://github.com/sindresorhus/ava/commit/945dbea40a6a500a24d54ebcc7e10dcd75647fc7), there's no longer any `power-assert` output.

``` js
import test from 'ava';

test(t => {
    t.ok(false);
});
```

```
  1 failed

  1. [anonymous]
  AssertionError: false == true
    at Test.fn (/Users/sindresorhus/dev/md5-hex/test.js:4:4)
```
