Update docs on Tilt::Template in Asset Pipeline guide

There have been some changes in the Tilt::Template protocol, which weren't updated in the guide on Asset Pipeline. I couldn't run the code sample without running into errors related to these changes.

* The `Template#prepare` method now needs to be implemented as well. It throws a `NotImplementedError` when it's not. See [this line in the Tilt::Template class](https://github.com/rtomayko/tilt/blob/master/lib/tilt/template.rb#L143)
* The original source is no longer available in the `@code` instance variable, but rather in the [`data` reader method](https://github.com/rtomayko/tilt/blob/master/lib/tilt/template.rb#L15).

This PR updates the docs to reflect these changes.