browsertests: bundle install gems in shared dir

Save up time when running `bundle install` by using a shared directory
which is common to all jobs running on a slave.  We did so with change
https://gerrit.wikimedia.org/r/#/c/112682/ for the other browser test
but missed the browsertests-qa-* jobs.

Reference: https://github.com/bundler/bundler/issues/2856

Updates jobs:

    qa-browsertests-build
    qa-browsertests-postmerge
    qa-browsertests-run

Change-Id: If71a91f1c89c5ff57b7ce77d9f87445dae896656