adding Windows 8.1 platform, so we can use it in saucelabs for IE11 testing

Author: lukeis

Diff for: java/client/src/org/openqa/selenium/Platform.java

@@ -71,6 +71,13 @@ public boolean is(Platform compareWith) {
     }
   },
 
+  WIN8_1("Windows 8.1", "win8.1") {
+    @Override
+    public boolean is(Platform compareWith) {
+      return compareWith == WINDOWS || compareWith == WIN8_1;
+    }
+  },
+
   MAC("mac", "darwin") {},
 
   /**