Revert "Require ActiveRecord as development dependency"

This reverts commit ba04f711e5b5aee98af96cc64f94fc5654141833
and commit 5bc195eee0865c076b485e59ffc9fef72350bc1b.

I can't find how to get Travis-CI to accept the ActiveRecord dependency
status change. Below are the last failing builds, and a few other
can be found in the Travis history - any help with this issue
would be greatly appreciated!

Failing builds (and error messages):
https://travis-ci.org/gonzalo-bulnes/simple_token_authentication/builds/37118588
https://travis-ci.org/gonzalo-bulnes/simple_token_authentication/builds/37118949
Do not add adapters documentation in README

Let's create another adapter and use the adapters interface
before making its documentation visible.
Meanwhile, the specs do provide enough details for anyone
interested to implement adapters.