Handle Semaphore builds without finished_at data.
Conditionally show Semaphore build duration in summary message.
Remove test route and sample payloads.

This is going to be replaced by documentation on testing webhooks with
`curl` and sample payload files.
Pass raw body to the webhook adapters.
Update the Semaphore adapter to parse the raw body as JSON.

This fixes the work-around added by commit 66c357eb.
Add documentation for testing the app with sample payload files.