Build: Remove dates from copyright notice

I'm not too sure about the change in Gruntfile.js, so please let me know if you would like it to be different, e,g, whether the jquery.org/license should be on the same line as the Copyright notice, whether it should be https://jquery.org/ or https://jquery.org/license also, or anything else.
Why the change from `(c)` to `Copyright`? You didn't do the same in the Sizzle PR: https://github.com/jquery/sizzle/pull/307/files

This is the banner put in the minified files, e.g. https://code.jquery.com/jquery-2.1.3.min.js