Use ruby 1.9.3

This is a requirement of current versions of Capybara.
Upgrade Capybara and Poltergeist

Upgraded to current versions, and fixed test failures relating to changes in behaviour in Capybara (some of the assertions are now more strict, and it only inspects visible elements by default).
Merge pull request #375 from alphagov/upgrade_capybara

Upgrade Capybara and Poltergeist