Add commit changes to notifier (take 2)

This follows #95 after dealing with a few issues.

The `compare_link` in the Slack notifier is now properly generated.

I rolled back the query-based implementation of "last known revision" in favor of the redis key implementation I initially had: ` Heaven.redis.get("#{name_with_owner}-production-revision")`

This key is set per repo/environment pair after a successful deployment.

Seems more straightforward than depending upon Status webhooks (which I currently have disabled anyway due to #103) or scraping the Deployments API's `deployment_statuses` resource but I'm definitely open to suggestions.

The commit change notifications can be controlled with two environment variables:

```
export DELIVER_CHANGES='true' # enabled commit changes
export COMMIT_CHANGE_LIMIT=5  # limit commit changes to N commits
```