fix filter json

The sensu server looks for keyword "filters" in the configs, not "filter".
see: https://github.com/sensu/sensu/blob/master/lib/sensu/server.rb#L116
and: https://github.com/sensu/sensu-settings/blob/master/lib/sensu/settings/constants.rb#L3
Merge pull request #267 from patrick-minted/fix_filter_lib

fix filter json