Support HEROKU_SLUG_COMMIT environment variable

[asgrim]

The docs state:

• If you are using Heroku, enable Dyno Metadata.
  This adds a HEROKU_SLUG_COMMIT environment variable to your dynos, which Scout then
  associates with deploys.

I don't think this is currently supported, so presumably we should add this as a feature.

[cschneid]

Yes, this is how the obtaining the SHA for Deploy tracking works on heroku. In the python agent, we look at revision_sha config first, then check this env var.