results beyond - holdings only search

setting [Holdings Only](http://api.summon.serialssolutions.com/help/api/search/parameters/holdings-only) to true when doing the Summon search seems to fix the problem of non-launchpad items showing up in the search results. I'm not sure why I was explicitly setting it to false before. This brings Launchpad in line with Obento's book search, which also [sets it to true](https://github.com/gwu-libraries/obento/blob/master/obi/obi/local_settings.py.template#L77).