Issue #88: Rollbar Deployment Actor

This actor interacts with the Rollbar API to send 'Deploy' messages
during your code deployments. Dry mode validates the API access by
reaching out and getting a list of projects with the API key. Wet
(non-dry) mode actually sends the Deploy API call.

https://rollbar.com/docs/deploys_other/
https://rollbar.com/docs/api/projects/#list-your-projects
Merge pull request #132 from Nextdoor/issue_88

Issue #88: Rollbar Deployment Actor