Fix errors in tests and make the biuld to be green

I tried to run tests on my local machine where all necessary developer's tools were installed (WAMP, Composer, Codeception, etc.). And I got these errors, which I've tried to fix:

* Fatal error: **Class 'Guzzle\Http\Client' not found** in ./tests/_helpers/MailHelper.php on line 30
* Catchable fatal error: **Argument 1 passed to GuzzleHttp\Client::__construct() must be of the type array, string given**, called in ./tests/_helpers/MailHelper.php on line 30 and defined in vendor/guzzlehttp/guzzle/src/Client.php on line 74
* [yii\base\InvalidConfigException] **The configuration for the "mail" component must contain a "class" element** 
* Fatal error: **Call to undefined method GuzzleHttp\Message\Response::send()** in /home/travis/build/bevalorous/yii2-user/tests/_helpers/MailHelper.php (on line 38, 48, 69, 92)

Now the build of the code fixed by me works in Travis CI: https://travis-ci.org/bevalorous/yii2-user.

P.S. Sorry for possible confusion of my commits, I've had very little experience with GitHub. Thank you for your job, Yii2-User is very good!