Fix redis deprecation warnings

[lavaturtle]

This fixes deprecation warnings, seen when running the specs in this gem or using it, that look like this:

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

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

should be replaced by

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

(called from /home/grey/code/sidekiq-grouping/lib/sidekiq/grouping/redis.rb:16:in `block in push_msg'}

Also: When I first tried to run the specs, I ran into an error that looked like rails/rails#43851, so this adds a require line that fixes the error.

[houndci-bot]

Style/FrozenStringLiteralComment: Missing magic comment # frozen_string_literal: true.

[houndci-bot]

Style/StringLiterals: Prefer double-quoted strings unless you need single quotes to avoid extra backslashes for escaping.

[reinaris]

@gzigzigzeo is this worth merging? Or do we need to fix the hound comments?

[mrexox]

@gzigzigzeo , what do you think about this PR? This change seems useful especially for the future versions of Redis.

[gzigzigzeo]

Thank you!