Added support for acceptSslCert capability in BrowserStack

Ref. #237

src/Behat/MinkExtension/ServiceContainer/Driver/BrowserStackFactory.php

@@ -62,6 +62,7 @@ class BrowserStackFactory extends Selenium2Factory
                 ->booleanNode('browserstack-debug')->end()
                 ->booleanNode('browserstack-tunnel')->end()
                 ->booleanNode('emulator')->end()
+                ->booleanNode('acceptSslCert')->end()
             ->end()
         ;