Address Slack SDK warning

[mfitz]

When posting messages via the Slack API, the slack-sdk gives this warning:

UserWarning: The top-level `text` argument is missing in the request payload for a chat.postMessage call - It's a 
best practice to always provide a `text` argument when posting a message. The `text` argument is used in places
where content cannot be rendered such as: system push notifications, assistive technology such as screen readers,
etc.
  warnings.warn(missing_text_message, UserWarning)