set version to v1.3.1 for release

Author: gremmyz

src/main/java/seedu/connectus/MainApp.java

@@ -36,7 +36,7 @@
  */
 public class MainApp extends Application {
 
-    public static final Version VERSION = new Version(1, 3, 0, true);
+    public static final Version VERSION = new Version(1, 3, 1, true);
 
     private static final Logger logger = LogsCenter.getLogger(MainApp.class);