Add scope without_deleted

Callbacks are countable, use several instead of few to reference them

Add a without_deleted scope to manually exclude paranoia-deleted records, which is essentially the paranoia_scope.
Useful when using the option :without_default_scope or the unscoping of a paranoia-model was required.
Aliased it to the paranoia_scope as opposed to the existing with_deleted scope.

Rails 5 Support, Require ruby 2.0

Just for consitency. alias_method is easier to reason about than
only_deleted.

Also updates ruby versions

In rails 4.x this passed around a set of arel conditions, in rails 5 it
passes around an AR::Relation and uses where().

See rails/rails@5f521cbff3c8a46cfc8e16b234294a20c285d00d

It is unnecessary to test the AR internals here. Testing the
where_values_hash should be sufficient.

We are switching to support of ruby 2.0 and greater. We will only
support jruby OVER 9000 for feature parity.

alias defines these on ActiveRecord::Base itself, which is not what was
intended. This behaviour previous differed between MRI and jruby.

Added without_default_scope option

Fix typo in readme

Update location of acts_as_paranoid repository

Fix grammar and typos in README

Adds changes in 2.1.5 and adds placeholders for missing release notes.

Fix really_destroy! for ruby 2.3

For some reason in ruby 2.3 this definition was interpreted as private.
There was no reason to be defining it where it was, so I've moved it
into the paranoia module.

Use TravisCI's container infrastructure

This makes the startup time for tests much faster.

This also fixes an issue where bundle install would fail because the
version on travis's legacy infrastructure is too old.

See: https://github.com/bundler/bundler/issues/3558

We're still looking for people to help, but a maintainer now exists