Prepare release 3.1.2

Author: Godin

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

@@ -11,7 +11,7 @@
 
 <h1>EclEmma Change Log</h1>
 
-<h2>Version 3.1.2 (not yet released)</h2>
+<h2>Version 3.1.2 (2019/03/12)</h2>
 
 <ul>
   <li>Upgrade to JaCoCo 0.8.3 (Eclipse Bug 543890).</li>