Add the :os_account_meta_temp_url_key to the carrierwave fog credentials hash

Hi,
is there any particular reason why the :os_account_meta_temp_url_key was removed in the a9s_swift gem?
In order to create temporary URLs for uploaded files, I had to add the lines like in the pull request and use the gem from the /vendor directory, which is kinda cumbersome.
In the setup instructions in [this anynines blog post](https://blog.anynines.com/how-to-use-the-anynines-swift-service-with-your-app) the key is still there...  
Thanks in advance