fix(runner): gracefully shutdown browsers after test

Related: https://github.com/angular/protractor/issues/1072
Instead of repeating calls to `gracefulShutdown()`, how about doing:

```js
  }).then(function() {
    var passed = testResult.failedCount === 0;
    var exitCode = passed ? 0 : 1;
    return self.exit_(exitCode);
  }).fin(function() {
   return gracefulShutdown(driver);
  });
```

The actual return value (or reject reason) of the promise will be from the previous then-able section. 
i.e. the exit code should be returned properly.