Don't allow caching of coffee requires to be overridden

Previously coffeestack was requiring a different version of coffeescript
than atom was, which had a side effect of re-registering the extension
handler for .coffee requires. This disabled coffee-cache, which made
require really slow again.

static/index.html

@@ -9,6 +9,10 @@
       try {
         require('coffee-script');
         require('coffee-cache').setCacheDir('/tmp/atom-coffee-cache');
+        Object.defineProperty(require.extensions, '.coffee', {
+          writable: false,
+          value: require.extensions['.coffee']
+        });
         require(currentWindow.loadSettings.bootstrapScript);
         currentWindow.emit('window:loaded');
       }