Adjust ping worker logger level to debug when offline.

[dblock]

Following up on https://github.com/slack-ruby/slack-ruby-client/pull/279 adjusting the ping logger level for the offline case to debug. Together with https://github.com/slack-ruby/slack-ruby-bot/pull/229 suppresses a lot of logger noise.

[dangerpr-bot]

	1 Warning
:warning:	There’re library changes, but not tests. That’s OK as long as you’re refactoring existing code.

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