Update testem version to fix  assertions

When looking at ember-cli, the first thing I did was see what testing was like out of the box. I started out with a simple assertion on `true` which fails.

_tests/unit/truth-test.js_
```javascript
import { test } from 'ember-qunit';

test('true is not false', function() {
  expect(2);
  equal(1, 2, 'these numbers are not equal');
  equal(true, false, 'true is not false');
});
```

It turns out this was an [issue with testem](https://github.com/airportyh/testem/issues/354) and the default tap reporter which has [now been fixed](https://github.com/airportyh/testem/commit/05d9b66f1835db9affd0305a0c07715aec2d626e).

_(I wasn't sure how to add this type of test directly to ember-cli or I would have included it in the pull request.)_