Fixed the configuration for Selenium2 capabilities

Some capabilities need to have a dash in their name to match the name
expected by Selenium2 (which uses a mix of camel cased and dashed names).
This fix makes the MinkExtension incompatible with Symfony 2.0 and 2.1 but
they already reached their end of maintenance anyway.
Closes #91

src/Behat/MinkExtension/Extension.php

@@ -271,6 +271,7 @@ class Extension implements ExtensionInterface
                             defaultValue(isset($config['selenium2']['browser']) ? $config['selenium2']['browser'] : '%behat.mink.browser_name%')->
                         end()->
                         arrayNode('capabilities')->
+                            normalizeKeys(false)->
                             children()->
                                 scalarNode('browserName')->
                                     defaultValue(isset($config['selenium2']['capabilities']['browserName']) ? $config['selenium2']['capabilities']['browserName'] : 'firefox')->