Use Resque::DataStore#reconnect for redis-rb v5.0

[mishina2228]

This PR will fix a part of #750.

Redis::Client#reconnect has been removed from redis-rb v5.0.0.
This PR replaces it with Resque::DataStore#reconnect.
It is available since Resque v1.27.0, so it is backward compatible.

• Resque v1.27.0
  https://github.com/resque/resque/blob/v1.27.0/lib/resque/data_store.rb#L80-L83

      # Force a reconnect to Redis.
      def reconnect
        @redis.client.reconnect
      end

• Resque v2.4.0
  https://github.com/resque/resque/blob/v2.4.0/lib/resque/data_store.rb#L79-L85

      # Force a reconnect to Redis without closing the connection in the parent
      # process after a fork.
      def reconnect
        @redis.disconnect!
        @redis.ping
        nil
      end

[mishina2228]

Test failure is not directly related to this PR.
I created #757 for it.

[mishina2228]

@PatrickTulskie @iloveitaly
If you have a chance, would you mind review this PR? Thanks!

[PatrickTulskie]

LGTM. Will work on a release for later today.

[PatrickTulskie]

Actually going to close this since it looks like these changes are in #757 and that is passing CI... also it would have a merge conflict if I did this one first. Thanks!

[mishina2228]

Thank you! and sorry for inconvenience caused by the conflict. 🙇
I didn't include the README change in #757, so I will submit another PR.

[mishina2228]

I've just submitted #760.

[PatrickTulskie]

Gotcha. 4.7.0 has been released with this and the readme update. Thanks for helping out!