Fixed a huge bug

I have changed occurs_at in occurrences table from timestamp to datetime because for some reason when you would toggle the payment_received or offer_sent field, the occurs_at field would be set to current date for some reason.

If you are upgrading your application you need to do the following:

# This will drop the whole table for occurrences and 
# create a new table with correct occurs_at column type
php artisan migrate

# This will repopulate the table with occurrences
php artisan occurrences:spawn