if query finds no row in the database, create a new one instead

fixes regression introduced in fc9ada9b44478d5638d07f1090fda6d7b0c5e771
remove unnecessary else
further simplify get_or_create

we now call have only one `model(**attrs)` call in this function
Merge pull request #185 from goodscloud/fix-for-get_or_create

if query finds no row in the database, create a new one instead