setting Sidekiq::Logging.logger to nil unless $VERBOSE is true

Hello,

We got quite a number of emails from cron since sidekiq was logging to stdout...

```
Date: Tue, 4 Feb 2014 20:19:33 -0800
From: Cron Daemon <snip>
To: "snip" <snip>
Subject: Cron <snip@snip> /usr/bin/scout <snip>

2014-02-05T04:19:31Z 24124 TID-a5wxk INFO: Sidekiq client with redis options {:url=>"redis://<snip>", :namespace=>"<snip>"}
```

This change sets Sidekiq::Logging.logger to nil, which suppresses the INFO messages, unless $VERBOSE is set to true.