Language screenshot jobs run in serial now

Changed job:

browsertests-VisualEditor-language-screenshot-linux-firefox-sauce

See also:

https://gerrit.wikimedia.org/r/#/c/148368/2/modules/ve-mw/tests/browser/upload.rb,cm

Change-Id: I02acc22ee4eb25e1bd0cb8b76f4b6189e5007630