Fix known blank page issue in Safari

There's been a weird edge-case caching issue with Express apps ever
since Safari 7 where users can see blank pages.

This change adds the "jumanji" dependency to fix the issue. Details
about the issue are in its README: https://github.com/Dakuan/jumanji
Merge pull request #128 from ericf/safari-blank-page

Fix known blank page issue in Safari