Updated app.yml and _config.yml to improve build speed

Essentially the same work as this (https://github.com/Polymer/docs/commit/1ec244afca87b70b3195b9d4a27627ecda92119c). I'm not sure if we could just cherry pick that commit because there are some slight differences in the `app.yml` file (version number, for instance).

What say you @ebidel ?