add depend_on_asset to asset-path.js

[thedaniel]

This might not be necessary, but it will ensure asset-path is recompiled by the asset pipeline when the swf file changes, as well as prevent dependency errors being raised by https://github.com/schneems/sprockets_better_errors

[myitcv]

Thanks - given that I don't think this is critical, it will go in the next release