No Longer loading environment and release from environment variable

The [old Python lib loaded `environment` and `release` from the environment](https://github.com/getsentry/raven-python/blob/03559bb05fd963e2be96372ae89fb0bce751d26d/raven/base.py#L198-L202) however this lib does not appear to do that. Further there is no mention of that change in the docs nor a clear suggestion for a migration path.