Use upstream sense community-plugins

We originally forked this since it didn’t support alerting on when a
value dipped below a certain value. This functionality is now available
in the upstream version, so use that and take advantage of other
development goodies in the upstream version, such as checking SSL cert
expiry.
Fix lint
Drop our flags

This was functionality added in our fork. Not sure it’s still
applicable? Rob suggests we test empirically in a safe environment.
Merge pull request #474 from alphagov/feature/use-upstream-sensu-community-plugins

Use upstream sense community-plugins