Merge pull request #10 from MattFenelon/2-4-support

Upgrade to spree's master branch version - 2.4.
Account for Shipping being displayed later in checkout flow

For some reason these specs started failing due to the shipping charge being shown later than the spec expected.
Unsure if this is a bug I inadvertently introduced into the spec or if Spree has changed its behavior, but more likely the former.
Test Cases 2b and 6 are passing again.
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.
remove unnecessary file
update VERSIONFILE to reflect current branches
add .DS_Store to ignore file
update SPREE_TAXCLOUD_VERSION
Add address validation to TODOs
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.