Upgrade qa slaves to latest bower 1.8.2

The registry http://bower.herokuapp.com is now down because deprecated. Returned http status code is 410 (gone).

As a direct consequences, many builds are failing:

• https://qa.nuxeo.org/jenkins/job/master/job/addons_nuxeo-web-ui-master/287/
• https://qa.nuxeo.org/jenkins/job/master/job/addons_nuxeo-travel-expenses-master/494/
• https://qa.nuxeo.org/jenkins/job/master/job/addons_nuxeo-platform-spreadsheet-master/853/

See https://twitter.com/bower/status/918073147789889536