Marked as #plugin:broken-plugin. All functionality except the CSS theme to draw a box around collapsed-details is built-in now.
If you think you need this for a import/migration, try doing this instead:
rake posts:remap'[\[spoiler\],\[details=Spoiler\]]'
rake posts:remap'[\[spoiler=,\[details=]'
rake posts:remap'[\[/spoiler\],\[/details\]]'