Install PhantomJS 1.9.7 from our PPA

This upgrades PhantomJS from 1.9.1 to 1.9.7, in line with CI.

- https://github.com/alphagov/ci-puppet/pull/172
- https://github.com/alphagov/ci-puppet/pull/173
Merge pull request #440 from alphagov/install-phantomjs-from-launchpad

Install PhantomJS 1.9.7 from our PPA