Bust spartacus asset caches (bug 1014564)

https://bugzilla.mozilla.org/show_bug.cgi?id=1014564

Set the spartacus build ID so spartacus can bust translations and use it as a query to bust main.js.
SPARTACUS_STATIC does not exist in `webpay/settings/base.py` it should exist there with a sane default for developers