Fix deprecated uses of `Redis#pipelined`

[casperisfine]

Context: https://github.com/redis/redis-rb/pull/1059

The following is deprecated

redis.pipelined do
  redis.get(key)
end

And should be rewritten as:

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

Functionally it makes no difference.

[OsamaSayegh]

Thank you @casperisfine!