Only test Opera 12 once

Looks like Sauce Labs is not going to have tests running on later
versions of Opera, probably because it switched to Chromium/Blink.
Removing it from the builds so we reduce the time it takes.

Gruntfile.js

@@ -118,9 +118,6 @@ module.exports = function (grunt) {
             {
               browserName: 'opera',
               version: '12'
-            },
-            {
-              browserName: 'opera'
             }
           ]
         }