Update FeatureContext.php

I was having tearDown issues till I fix this. Here was the php error.

PHP Fatal error:  Cannot access private property Behat\Mink\Driver\Selenium2Driver::$wdSession in /opt/testing_bot/behat-mink-testingbot-example/features/bootstrap/FeatureContext.php on line 66
Merge pull request #2 from alnutile/master

Update FeatureContext.php