Adding SS 3.1 support. Changing minimum requirement to 3.1.

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