Use existing webhooks

https://trello.com/c/9I7EgBue/372-fix-subscription-issue

* Subscriptions were failing when webhook already existed, this change
* fixes the issue by using the existing webhook.
Add pull request info to Build model

* add `commit_sha` and `pull_request_number` attibutes
* extract stub helpers in pull_request_spec
* dry up tests
Handle nil violations