Add support for env_dir instead of user-env-compile

user-env-compile has been deprecated and replaced
with a new 3rd env_dir argument. This change updates
the buildpack to extract keys from the ENV_DIR while
maintaining backward compatibility with apps still
using user-env-compile.

Ref:
https://devcenter.heroku.com/changelog-items/416
https://devcenter.heroku.com/articles/buildpack-api#bin-compile