Remove support for PhantomJS/Poltergeist
Remove support for PhantomJS/Poltergeist

The tests now run fine in Chrome, so we can drop support for PhantomJS/Poltergeist, which are very obsolete and don't work with the latest Capybara version.

Re-instate the original meaning of `BROWSER=true` to run the tests in non-headless mode.

alphagov/digitalmarketplace-functional-tests,alphagov/digitalmarketplace-functional-tests,alphagov/digitalmarketplace-functional-tests