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

Touching record when restored.

Using guard clauses.

Simplify description in README

I had a hard time comprehending the old wording. I think this is easier to read.

Simplify test setup

* Rely on bundler/setup to setup the load paths

Fixed issue #258: uniqueness validator now works with custom active c…

Introduce #paranoia_destroy_attributes and #paranoia_restore_attributes extension points

Use update_columns rather than touch to update the record, for
generality

Unlike touch, update_columns does not create a transaction for
itself, so we need to add the record to the transaction, if present.
If there is not a current_transaction, the add is a no-op.

This all means that delete will not invoke a transaction or run the
after_commit callbacks unless called from within one, which is
consistent with the Rails docs and the behavior of
ActiveRecord::Base#delete.

Fix uniqueness validator for non-paranoid associations

real_destroy callback

Fix for #122, #121 and #175 on Rails 4 branch

Ruby 1.9's version of `Object.const_get` does not support namespaced constants, so this commit replaces it with `ActiveSupport`'s `#constantize` method.

The tests were failing running JRuby on 1.9 mode. CRuby was not failing because it is not tested against 1.9.

Fixes #114

Fix association class name for namespaced has one model lookups