Fixing SS 3.1 support. Changing minimum requirement to 3.1.

In SilverStripe 3.1 `Object::has_extension()` doesn't have an argument fulltextsearch was relying on. As a result, you can't save a page with the fulltextsearch module installed. This fixes that to work with SilverStripe 3.1 correctly, however it also removes support for 3.0 at the same time.

Given we don't have any official release of this module, is this bump in minimum requirements to 3.1 acceptable?