Enable the no-else-return ESLint rule

Using `else` after `return` is not necessary, and can often lead to unnecessarily cluttered code. By using the `no-else-return` rule in ESLint we can avoid this pattern, see http://eslint.org/docs/rules/no-else-return.

src/core/obj.js

@@ -1585,9 +1585,8 @@ var FileSpec = (function FileSpecClosure() {
       return dict.get('Mac');
     } else if (dict.has('DOS')) {
       return dict.get('DOS');
-    } else {
-      return null;
     }
+    return null;
   }
 
   FileSpec.prototype = {