Symbolization of passes options

Forcing key symbolization for passes options makes possible to configure the gem changing only the gitlab.yml file using standard formatting:

provider:
   - { name: 'crowd', args: { crowd_server_url: 'http://<CROWD_SERVER_IP:PORT>/crowd', application_name: "<APP_NAME>", application_password: "<APP_PASSWORD>", title: '<YOUR_DOMAIN_NAME> Crowd' } }

Also supports the older configurations.

Tested with Gitlab 6.6.0 and omiauth_crowd 2.2.2.