Change en.js to be an empty file, rather than pointing to en-ca.js.

Summary:
This means the default khanacademy site (with no lang= parameter) will
use the default moment-js strings, which is what we want since those
default strings are for en-US.

Test Plan:
Ran a dev-appserver and logged in as a user who coaches a student, and
visited
   localhost:8080/class_profile/progress-report
I clicked on the 'all time' dropdown in the top right, and selected
custom range', and saw that the range said 'Start: June 9 2013',
whereas before this change it said 'Start: 9 June, 2013'.

Reviewers: joshnetterfield, chris, khirasaki

Reviewed By: khirasaki

CC: james

Differential Revision: http://phabricator.khanacademy.org/D2623