Improved the guessing of capabilities on Travis

The logic is not duplicated anymore between BrowserStack and Travis, and
user-defined capabilities now win over guessed capabilities.
Refs #140

src/Behat/MinkExtension/ServiceContainer/Driver/Selenium2Factory.php

@@ -60,9 +60,21 @@ class Selenium2Factory implements DriverFactory
         $extraCapabilities = $config['capabilities']['extra_capabilities'];
         unset($config['capabilities']['extra_capabilities']);
 
+        if (getenv('TRAVIS_JOB_NUMBER')) {
+            $guessedCapabilities = array(
+                'tunnel-identifier' => getenv('TRAVIS_JOB_NUMBER'),
+                'build' => getenv('TRAVIS_BUILD_NUMBER'),
+                'tags' => array('Travis-CI', 'PHP '.phpversion()),
+            );
+        } else {
+            $guessedCapabilities = array(
+                'tags' => array(php_uname('n'), 'PHP '.phpversion()),
+            );
+        }
+
         return new Definition('Behat\Mink\Driver\Selenium2Driver', array(
             $config['browser'],
-            array_replace($extraCapabilities, $config['capabilities']),
+            array_replace($extraCapabilities, $guessedCapabilities, $config['capabilities']),
             $config['wd_host'],
         ));
     }