license: update years and authors

This obviously hasn't been updated since the site started.
There's not much point in trying to list all of the authors,
so I'll just say "and others". Anybody who really wants to
know can use `git log`.

(I doubt anybody cares that much at all, but since we're now
linking the license file from the "about" page, it may get
more attention).

MIT-LICENSE.txt

@@ -1,4 +1,4 @@
-Copyright (c) 2012 Scott Chacon
+Copyright (c) 2012-2018 Scott Chacon and others
 
 Permission is hereby granted, free of charge, to any person obtaining
 a copy of this software and associated documentation files (the