Ensure no old gems are left behind between test runs.

The issue is thus:
1. We run a job on a jenkins-worker that includes a gem upgrade to sass 3.3. This produces deprecation warnings
** CC @andy-armstrong on that issue, related to #3462. I'm guessing those deprecation warnings will have to be addressed.
2. Then we run another job on the same jenkins-worker that is using the old gem (say, running on master)
** However, in this second run, we still get the deprecation warnings. This is because the previously-upgraded gems are still left behind in the rbenv.

@jzoldak @clytwynec Can you take a look?