fix non-static method called statically

Hi there,
thanks a lot for writing this plugin! I found a small problem in class-wc-dependencies.php and also added support for qtranslate-xp (https://wordpress.org/plugins/qtranslate-xp/).
For the qtranslate-xp I added extra 'function_exists' conditionals everywhere, since qtranslate-xp uses a different function namespace (ppqtrans*) than (m)qtranslate.