Version-related cleanup

Rebase of #412

This removes the assumption from `really_destroy!` that there is
only one column value, the deletion date, that needs to be updated
on a model that is being `really_destroy!`ed. Instead, use the
defined `paranoia_destroy_attributes` as they are used in `destroy`
and other calls.

This prevents an issue where the DBMS will complain when trying to
`really_destroy!` one of these models, given that paranoia is
attempting to insert a DateTime value into a boolean (or otherly-
typed) column.

Fixes #326

Update .travis.yml

Removed the pessimistic dependency requirements.
Updated Ruby versions.
Test paranoia with Rails 5.1

⚠ instance variable @_disable_counter_cache not initialized

This `defined?(super)` check was introduced for Rails 4.1 support
https://github.com/rubysherpas/paranoia/pull/390#issuecomment-284819747
but we've stopped supporting Rails 4.1 already
https://github.com/rubysherpas/paranoia/pull/393#issuecomment-289227091

Version 2.3.1

bump active record max version

Version 2.3.0

Test with rails 5.1 and ruby 2.4.0

Changelog: Add missing bang in `#really_destroy!`

add version 1.4 of the Contributor Covenant

Fix counter cache with double destroy, really_destroy, and restore

Added association not soft destroyed validator

Fix UniquenessParanoiaValidator on rails 5.1.0.beta1

Add recovery window feature

add changes for 2.2.1 and bump version

Fixes a problem of ambiguous table names when using only_deleted meth…