Rake exits non-zero when specs fail!
Don't reference a let!() var in a before(:all) block.

According to RSpec:

> This is deprecated behavior that will not be supported in RSpec 3.
>
> `let` and `subject` declarations are not intended to be called
> in a `before(:all)` hook, as they exist to define state that
> is reset between each example, while `before(:all)` exists to
> define state that is shared across examples in an example group.
DatabaseStrategy#switch! uses `update_attributes!`.

This is less procedural, and is slightly simpler to test.
Simpler double/stub declaration in database_strategy_spec.
database_strategy_spec fixed since ActiveRecord usage changes.

Pull request #8 included 51cab990ae1ea609a703b8ffb9fc65596f7e23a3 which
uses a different ActiveRecord API, requiring this spec to be updated.

I have begun to introduce RSpec's newer expect(subject).to(...) and
allow(subject).to(...) style expectations.