Update license to GPL v3

Because the GPL is "viral", legally the larger project is required
to use it also. This is kind of annoying, and there could potentially
be ways to work around it (e.g. have a prep script that downloads
`twinejs` on a user's local machine rather than redistributing it).
I'd rather just change `twinejs-app` to use GPL (and so satisfy
redistribution requirements) than to do something weaselly like that,
though.