Updated license to version 2.

Version 1 of the license didn't make sense: it required a Spine license to use the Spine Runtimes at all. This means if you used the Spine Runtimes in your application, anyone who purchased a copy of your application also needed a Spine license to use the runtimes. This was not the intent!

Version 2 of the license fixes this by allowing anyone to use the Spine Runtimes but only for internal use. When you purchase Spine, the Spine license (http://esotericsoftware.com/spine/files/license.txt) grants you the right to make and distribute derivative works of the Spine Runtimes. Now it makes sense: making and distributing applications with the Spine Runtimes requires a Spine license, but anyone can run that software without needing a Spine license.

With this fixed, I hope we never need to change the license again!