Add paranoia_destroy and paranoia_delete aliases

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

This makes paranoia's code itself more explicit.

This should help users be more explicit about what action they want to
perform. My hope is that this will also make it easier to migrate off of
paranoia.

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