Update corehq/apps/smsbillables/models.py
Update corehq/apps/smsbillables/models.py

Co-authored-by: Daniel Miller <478aa1ed776ceaf3915ffaea32c34a302955f457@gmail.com>
dimagi/commcare-hq,dimagi/commcare-hq,dimagi/commcare-hq,dimagi/commcare-hq,dimagi/commcare-hq