Fixed prepareSession() if session not exists

If session not exists `SauceTestCommon::ReportBuild($this->getSessionId(), $this->getBuild())` throws exceptions (Exception: job_id is required
). So we should create session first.
Update src/Sauce/Sausage/WebDriverTestCase.php
Merge pull request #3 from bayandin/patch-1

Fixed prepareSession() if session not exists