Move away from deprecated use of #pipelined

[ltk]

This adds support for the latest version of redis-rb, and should work for redis-rb versions down to 3.0 (might want to confirm this).

Deprecation message:

Pipelining commands on a Redis instance is deprecated and will be removed in Redis 5.0.0.

redis.pipelined do
  redis.get("key")
end

should be replaced by

redis.pipelined do |pipeline|
  pipeline.get("key")
end