Make spree_tax_cloud play nice with other tax rates.

This stops spree_tax_cloud committing tax cloud transactions for orders that do not use the tax cloud calculator. Stores that ship into and outside the US are likely to have multiple tax rates, with only the US tax rate being applicable for orders into the US.
Upgrade to spree's master branch version - 2.4.
Switch the capybara driver to poltergeist.

It's not obvious how to configure the default driver on a headless server whereas poltergeist works in headless mode out of the box. Spree also uses poltergeist.
Merge branch '2-4-support'
Merge branch 'poltergeist'