qa/browsertests Gemfile has been moved

The Gemfile of qa/browsertests has been moved from the root of the
repository down to tests/browser https://gerrit.wikimedia.org/r/111997/

Point bundler to the Gemfile using:

 bundle install --gemfile=tests/browser/Gemfile

Adjust the archiver for the postmerge job, the .bundle path is now under
test/browser as well.

Updates jobs:

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

Change-Id: Ic0ab3c94cf1a5d9292742168f13e9006647dedbb