ENYO-1740, ENYO-1888: fsDropbox behind corporate proxy (4/5)

- the `dropbox-js` and `xhr2` author has made the necessary changes to
  make this library operate from behind a corporate proxy.
- this is change 4/5, containing an updated `dropbox-js` that can work
  behind proxies (from 0.8.1 to 0.9.1-beta2).

Enyo-DCO-1.1-Signed-off-by: Francois-Xavier KOWALSKI <francois-xavier.kowalski@hp.com>