Don't allow caching of coffee requires to be overridden in tasks

This is a follow-on to the previous commit which handled the index.html
case.

src/stdlib/task.coffee

@@ -16,6 +16,10 @@ class Task
     bootstrap = """
       require('coffee-script');
       require('coffee-cache').setCacheDir('/tmp/atom-coffee-cache');
+      Object.defineProperty(require.extensions, '.coffee', {
+        writable: false,
+        value: require.extensions['.coffee']
+      });
       require('task-bootstrap');
     """