Support deploying abitrary SHA to heroku

**So that** I can deploy known good builds in a CD pipeline
**As a** devops person configuring CI
**I want** to be able to specify a specific commit to deploy to heroku

Deployer should support providing a treeish which specifies which commit to push. Right now it's hardcoded to HEAD. 
https://github.com/moredip/heroku-headless/blob/master/lib/heroku-headless/deployer.rb#L100
