Update Shell to use Context.VERSION_ES6 by default

resolves https://github.com/mozilla/rhino/issues/1279

rhino-tools/src/main/java/org/mozilla/javascript/tools/shell/ShellContextFactory.java

@@ -13,7 +13,7 @@ import org.mozilla.javascript.ErrorReporter;
 public class ShellContextFactory extends ContextFactory {
     private boolean strictMode;
     private boolean warningAsError;
-    private int languageVersion = Context.VERSION_1_8;
+    private int languageVersion = Context.VERSION_ES6;
     private int optimizationLevel;
     private boolean generatingDebug;
     private boolean allowReservedKeywords = true;