Skip the synchronization in the auto-update check script

The auto-update check script is using https://nightly.nuxeo.com/nuxeo to bind an account and test the auto-updater. But if one added one or more synchronization roots, the auto-update script will not work as it kills the app after 60 seconds.

To bypass that, the script should use the synchronization-enabled = False option.

—

Also, if nightly is down (as it is so for 2 days now), then the check will never be good. Let's bypass the need for an annount by using the FORCE_USE_LATEST_VERSION=1 envar.

-> Will not apply that patch as we would loose critical checks like actions done via the CLI (new account, new sync root, sync root deletion, account removal).

To manually use the envar, add this code in the Travis-CI custom config:

env:
  global:
    - FORCE_USE_LATEST_VERSION=1