Bump license year
I feel like there's a 20xx joke to be made here...
The MIT License
-Copyright (c) 1997-2018, Carsten Sorensen and RGBDS contributors.
+Copyright (c) 1997-2020, Carsten Sorensen and RGBDS contributors.
Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to
