Require native-host.js so app communication works again

Test Plan:
Launch the app.
Go to a video.
Open the Web Inspector.
Tap the "Download" buttion.
Assert that you don't get errors related to
"updateNativeHostWithCategoryActions" and that the
video downloads (the download button should be
replaced with text indicating download progress).

Reviewers: mgp

Reviewed By: mgp

Differential Revision: http://phabricator.khanacademy.org/D11924