refactor MergeRequestTabs to es6 class syntax

app/assets/javascripts/merge_request.js

@@ -40,7 +40,7 @@
       if (window.mrTabs) {
         window.mrTabs.unbindEvents();
       }
-      window.mrTabs = new MergeRequestTabs(this.opts);
+      window.mrTabs = new gl.MergeRequestTabs(this.opts);
     };
 
     MergeRequest.prototype.showAllCommits = function() {

spec/javascripts/merge_request_tabs_spec.js

@@ -17,7 +17,7 @@
     };
     fixture.preload('merge_request_tabs.html');
     beforeEach(function() {
-      this["class"] = new MergeRequestTabs();
+      this["class"] = new gl.MergeRequestTabs();
       return this.spies = {
         ajax: spyOn($, 'ajax').and.callFake(function() {}),
         history: spyOn(history, 'replaceState').and.callFake(function() {})