qa-browsertests: always build before running tests

We cant really use artifacts since there is no way to finely select
which artifact we want. So we would always install the browsertests with
bundler before running them.

Note that the qa-browsertests-run job is not triggered because it is
painfully long and fails under phantomjs anyway.

Change-Id: I4ea2d0367dd64065728521675165f0edd459d5bb