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 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.

Ref:
https://devcenter.heroku.com/changelog-items/416
https://devcenter.heroku.com/articles/buildpack-api#bin-compile
Use https when specifying buildpack url
Remove `--stack cedar` option

This is now the default stack on Heroku
and does not need to be specified when
creating apps.
Merge pull request #6 from ryanbrainard/support-env-dir

Remove user-env-compile dependency