updating soft-delete flags

Thanks for the advice; I've now set the check to also work in earlier versions.

In unusual cases where the old flag variable and the new one are *both* set (for example if people have manually set softDelete in their model code but are using newer versions of laravel), I've made it so that the new flag overrides the old one - this is to make sure, for example, that calls to forceDelete are handled right.

Fixes #92.