Update CDNJS link for 1.1.5

Update the README to link to the most recent version of Phaser available from CDNJS.

It looks like https://github.com/cdnjs/cdnjs/pull/2686 has been merged.

Phaser 1.1.5 is now available online from CDNJS at:
http://cdnjs.cloudflare.com/ajax/libs/phaser/1.1.5/phaser.min.js