Allow rails 3.1

So the tests on rails3 don't pass at all (including on 3.2), but the same tests fail on 3.1 and 3.2.

Unfortunately, the latest version of this gem available for 3.1 suffers from https://github.com/radar/paranoia/issue/81. This patch will make fix that issue for rails 3.1 apps.
Please make this `~> 3.1`, by using `>=` you're actually allowing any future version of Rails, including a Rails 4, 5 or 6 version.