Fix after_commit for Rails 6

Activerecord 6 support

The versions we were running against were bumped, so we needed to update
which versions were being excluded as they were stale.

Rails master (soon to be 6.0.0) doesn't support versions before Ruby
2.5.

These versions of rails enforce this version constraint when loading the
sqlite adapter so we need to make sure that we always use a compatible
version.

Older versions of activerecord require sqlite ~> 1.3.6, but not in the
gemfile, the constraint is enforced when you load the adapter.

This allows us toe specify in the environment the version of the gem
we'd like to run against.

We are trying to test our application on rails 6.0.0.alpha but we can't install this because of the version lock
Probably this will be helpful that we can try and report any issues with edge rails

Add Ruby 2.6 to Travis CI.

Fix typo (`6.2.0` -> `5.2.0`)

Version 2.4.1

Improvements to rails 5.2 patches

Monkeypatch activerecord relations to work with rails >=5.2.0rc2

Test new ruby versions

Add a very soft deprecation notice

This isn't a change from how paranoia has been developed for the past
few years, but makes it more explicit.

I hope the recommendation against new projects using paranoia isn't too
controversial. I think anyone who has used this gem extensively has been
surprised by it many times. This is just the nature of replacing
ActiveRecord's behaviour. I believe paranoia does extremely well given
that it does that.

A lot of developers will reach for paranoia or acts_as_paranoid because
they're the first result they will find. However most users are just
looking to hide or disable some records, and they would do much better
to just toggle a flag rather than have paranoia override ActiveRecord's
behaviour on their model.

I hope linking to discard here isn't inappropriate, I believe it's the
best resource describe the caveats of paranoia as well as give a good
example of how to soft-delete without needing paranoia or
acts_as_paranoid.

Test with Rails 5.2

Version 2.4.0

Add paranoia_destroy and paranoia_delete aliases

Version-related cleanup