Prepare release 3.1.3

Author: Godin

org.eclipse.eclemma.doc/pages/changes.html

@@ -11,22 +11,19 @@
 
 <h1>EclEmma Change Log</h1>
 
-<h2>Version 3.1.3 (not yet released)</h2>
-
+<h2>Version 3.1.3 (2020/03/13)</h2>
 <ul>
   <li>Upgrade to JaCoCo 0.8.5 (Eclipse Bug 550852)</li>
   <li>EclEmma now distributed under the terms and conditions of the
       Eclipse Public License Version 2.0.</li>
 </ul>
 
 <h2>Version 3.1.2 (2019/03/12)</h2>
-
 <ul>
   <li>Upgrade to JaCoCo 0.8.3 (Eclipse Bug 543890).</li>
 </ul>
 
 <h2>Version 3.1.1 (2018/09/12)</h2>
-
 <ul>
   <li>Adjusted default colors in Dark theme (Eclipse Bug 533264).</li>
   <li>Upgrade to JaCoCo 0.8.2 that provides support for Java 11 (Eclipse Bug 537807).</li>