Fix file upload for VisualEditor language screenshots

We were installing gems built for Ruby 2.0, but then using the gems from Ruby 1.9.

The change updates all browsertests-* jobs, but the changed code will run only in browsertests-VisualEditor-language-screenshot-linux-firefox job.

Change-Id: Ie6fd61f3ec1e11dab4142f7987756ede6a131772