Update README with new edx-configuration branch
Update README with new edx-configuration branch

A new `hastexo_xblock` branch of edx-configuration has been created,
containing just the changes needed to for the hastexblock to work.
Update the README to reflect this.

hastexo/hastexo-xblock,arbrandes/hastexo-xblock,arbrandes/hastexo-xblock,hastexo/hastexo-xblock,arbrandes/hastexo-xblock,arbrandes/hastexo-xblock,hastexo/hastexo-xblock,hastexo/hastexo-xblock,arbrandes/hastexo-xblock