Fix formatting of “invalid choice” error message
Fix formatting of “invalid choice” error message

The `invalid_choice` error message was changed in Django by commit
ee77d4b25360a9fc050c32769a334fd69a011a63 to use a mapping key. Passing a
dictionary to the `invalid_choice` format string is now required.

stephenmcd/mezzanine,christianwgd/mezzanine,ryneeverett/mezzanine,dsanders11/mezzanine,christianwgd/mezzanine,ryneeverett/mezzanine,frankier/mezzanine,wbtuomela/mezzanine,readevalprint/mezzanine,frankier/mezzanine,ryneeverett/mezzanine,jerivas/mezzanine,molokov/mezzanine,jerivas/mezzanine,dsanders11/mezzanine,dsanders11/mezzanine,frankier/mezzanine,stephenmcd/mezzanine,jerivas/mezzanine,molokov/mezzanine,readevalprint/mezzanine,wbtuomela/mezzanine,stephenmcd/mezzanine,readevalprint/mezzanine,wbtuomela/mezzanine,christianwgd/mezzanine,molokov/mezzanine