Remove user-env-compile dependency

`user-env-compile` has been deprecated and replaced
with a new 3rd `env_dir` argument. This change updates
the buildpack to extract the `GITHUB_AUTH_TOKEN`
from the `env_dir` with a fallback to fetch it from `ENV`
for apps with `user-env-compile` still enabled. Once all apps
have been unflagged, the fallback can be removed. 
I can let you know when that happens.

Refs:
https://devcenter.heroku.com/changelog-items/416
https://devcenter.heroku.com/articles/buildpack-api#bin-compile

cc: @friism