Lua script only has one Redis KEY, the other variable should be ARGV

Since I'm a Redis Lua scripting newbie, it took some time for me to realize why KEYS (two of them) were used here. While using keys for both key and content works, content isn't really a Redis key and causes some confusion.

Official Redis dist.lock article also uses ARGV for the random value (http://redis.io/topics/distlock), so this might be a good correction. Great work BTW. I did some digging and I think this might be the only real node.js implementation of true Redlock algorithm.