change asset pipeline to not be built in source

When the `.asset_pipeline` is built in source, jekyll serve with the watch option (`-w`) goes into an infinite rebuild loop in my setup on OSX for [this site](https://github.com/stdio-ghana/stdio-ghana.github.io) (sorry I don't have a smaller example).

Moving where .asset_pipeline is created seems the simplest solution.  There's probably a better one hooking into the watch process, but I'm not familiar enough w/ Ruby + Jekyll to do that.