Fix merge button detection

Author: Fryguy

app/scripts/contentscript.js

@@ -4,7 +4,7 @@
   var changeMergeButtonState = function() {
     var $container = $('#js-repo-pjax-container');
     var issueTitle = $container.find('.js-issue-title').text();
-    var $buttonMerge = $container.find('.merge-message button.merge-branch-action');
+    var $buttonMerge = $container.find('.merge-message button.js-merge-branch-action');
     var disabled = false;
     var buttonHtml = '';