browsertests: enable Performance publisher

Enable the Jenkins Performance reporting plugin:

https://wiki.jenkins-ci.org/display/JENKINS/Performance+Plugin

Wikidata is using it to build dashboards of test execution time and
latency.  The plugins simply parse the Junit result.  Tobi Gritschacher
introduced it on the QA mailling list:

https://lists.wikimedia.org/pipermail/qa/2014-September/001972.html

Change-Id: I716f4e087ea4b7e2c60852419219cd37cc9f6be9