Update Composer SS Requirements & Add 3.0 Branch

[purplespider]

Just had some annoying issues after running a composer update on my sites, as the master of this module has been updated to support SS 3.1, so it no longer works on 3.0, but the composer.json file still says that only SS 3.* is required, so this caused composer to update this module on a 3.0 site breaking it. Had to manually restore an older version of the module from a backup, and remove it from composer.

To fix this can you please create a working 3.0 branch, and update the composer.json on the master branch to require SS ">=3.1.x-dev,<4.0".

Cheers!