Safe payloads

[jaxxstorm]

If trying to load events in slack or hipchat, the handler dies:

[2016-02-21 12:10:30 UTC] DEBUG: Closing connection to the Slack Real Time Messaging API.
/home/lbriggs/.rvm/gems/ruby-2.3.0/gems/lita-slack-1.7.2/lib/lita/adapters/slack/rtm_connection.rb:107:in `safe_payload_for': Cannot send payload greater than 16000 bytes. (ArgumentError)
    from /home/lbriggs/.rvm/gems/ruby-2.3.0/gems/lita-slack-1.7.2/lib/lita/adapters/slack/rtm_connection.rb:62:in `block (2 levels) in send_messages'
    from /home/lbriggs/.rvm/gems/ruby-2.3.0/gems/lita-slack-1.7.2/lib/lita/adapters/slack/event_loop.rb:10:in `block in defer'
    from /home/lbriggs/.rvm/gems/ruby-2.3.0/gems/eventmachine-1.0.9.1/lib/eventmachine.rb:1067:in `block in spawn_threadpool'