Add compatibility with the 4-arg version of is-master-down-by-addr

Redis 2.8.0 adopted the upstream (v3?) version of the sentinel protocol.  I can't claim to understand all the changes, but the net impact to redis-sentinel is that is-master-down-by-addr requires two additional args: the current time and the runid.

This is the quickest patch solution I could find to get back up and running after moving from 2.8.0-rc6 to 2.8.0.  I welcome more elegant solutions if there are any.