NEWS: Don't prefix version numbers with 'v'

It was only used in two instances, so get rid of it for
consistency's sake.

docs/news-2015.html.in

@@ -260,7 +260,7 @@
       </li>
     </ul>
 
-    <h3>v1.2.21: Nov 04 2015</h3>
+    <h3>1.2.21: Nov 04 2015</h3>
     <ul>
       <li>Documentation:<br/>
       docs: event impl. registration before hypervisor connection (Dominik Perpeet),<br/>

docs/news.html.in

@@ -16,7 +16,7 @@
     to gauge progress.
     </p>
 
-    <h3>v1.3.1: Jan 17 2016</h3>
+    <h3>1.3.1: Jan 17 2016</h3>
     <ul>
       <li>Features:<br/>
       Various improvements for the Xen libxl driver (Jim Fehlig),<br/>