Don't dispose domain object

Domain object disposal causes gulp process to be terminated prematurely when running on Windows and gulp output is piped to another process (pipe doesn't get a chance to pick up buffered data).

As per @isaacs, `dispose` is usually not required and will be deprecated soon. See [here](https://github.com/joyent/node/issues/5018) and [here](https://github.com/joyent/node/issues/4805#issuecomment-13862816).