Update composer.json

The first part of the Composer package name is supposed to indicate vendor, rather than the using it to indicate that this is a silverstripe module.

We're building a site that aggregates all the SilverStripe themes and modules (see http://extensions.andrewshort.name for a sneak peek) so we don't need to worry about them getting lost.

Creating your own vendor prefix minimises the change of naming conflicts, and will provide you a listing on http://extensions.andrewshort.name/vendors