update date casting

[cmgmyr]

This PR updates the model's $dates = [] to use $casts = []. This also drops the reference for the deleted_at field, which is redundant from using the SoftDeletes trait.

Anyone still using this package on Laravel versions 5.8, or under, will have to:

1. Create their own models
2. Extend the package models
3. Re-add the $dates = [...] property
4. Reference the new models in the config file

Fixes #393