The Webex Teams and Unify Circuit integrations didn't work correctly for
pipeline notifications, because they were using the method
`ChatMessage::PipelineMessage#pretext` which is overriden to always
return an empty string.

We can fix this by calling `#summary` instead, which is also what
`ChatMessage::BaseMessage#pretext` does by default. No other message
types override `#pretext` or `#summary`, so this change should be
backwards-compatible.