Rename snuffleupagus rule param from var_name to varname

I think there is an error in the current snuffleupagus config. We copy&pasted the rules from the readme and there was a typo (https://github.com/jvoisin/snuffleupagus/issues/312). Since the release 0.7.1 this throws a warning (https://github.com/jvoisin/snuffleupagus/pull/367).

I hope this will fix the warnings we currently see on some php setups. I couldn't test the changes on my local machine. So some tests need to be done.