lang/urweb: update to 20150520

ChangeLog:

• Change default behavior of client-side GUI event handlers: By default, events are now passed to handlers on parent DOM nodes as well, just like in normal JavaScript. Call [preventDefault] or [stopPropagation] to tweak that behavior. WARNING: This change may break backward compatibility!
• URIs specified with 'file' .urp directive are implicitly allowed to be referenced.
• New HTML tags: <fieldset>, <legend>
• New urweb-mode Emacs command: 'urweb-close-matching-tag'
• Bug fixes

PR: 202263
Submitted by: Timothy Beyer <beyert@cs.ucr.edu> (maintainer)