The django-storages library was upgraded to version 1.8 from version 1.4 as part of this commit and merged to master. One of the changes in version 1.7 is to not allow media location with a leading slash. But the configuration repo used to provision sandboxes declares the media location with a leading slash (c.f. edx_media_dir variable). This is breaking the sandbox provisioning with the error:
Install edxapp sandbox with master branch using configuration repo.