add strip_newlines filter for data in chapters.js

Aiming to fix #115 - it seems any jekyll version `>=2.5` throws a "stack level too deep" error when using the `jsonify` filter in [line 11 of chapters.js](https://github.com/maptime/maptime.github.io/blob/master/js/chapters.js#L11). Adding the filter `strip_newlines` fixes the issue for me, and hoping doesn't create any new issues for lesser versions of jekyll! Would love someone to test this out if they're `<2.5`.