Update copyright in license

This ensures that the year is up to date, and
that the copyright is assigned to the not-for-profit
Exercism, rather than the (now-defunct) Delaware
Corporation that we had in place as a temporary
measure for legal protection until we could
figure out how to structure things properly.

Author: Katrina Owen

LICENSE

@@ -1,6 +1,6 @@
 MIT License
 
-Copyright (c) 2017 Exercism, Inc
+Copyright (c) 2019 Exercism
 
 Permission is hereby granted, free of charge, to any person obtaining a copy
 of this software and associated documentation files (the "Software"), to deal