Fix Javascript tests (includes switching to ruby 1.9.3)

This upgrades capybara to 2.1, and switches from useing capybara-webkit to poltergeist (phantomjs driver).

Capybara 2.1 requires ruby 1.9.3, so this introduces a `.ruby-version` file to select ruby 1.9.3 in prod using the shiny new rbenv stuff.