Dont need to run rake test_app.

Conflicts:
	CONTRIBUTING.md
Update selenium-webdriver to run with latest Firefox, bump Capybara and
rspec-rails versions too.

Also in this commit are a number of changes to the test running documentation.

Conflicts:
	CONTRIBUTING.md
	README.md
	backend/spec/spec_helper.rb
	common_spree_dependencies.rb
	frontend/spec/spec_helper.rb
Run 'bundle check || bundle install' to ensure dependencies are installed before build.sh
Use jsuri for products/new prototype selector too

Related to #2248
Fix tax calculator with non-VAT taxes.

This is an issue with Spree line items and tax rate calulators.

The method used to subtract tax from a VAT did not play nicely
with non value added taxes.

Fixes #2870
Pass args to provider method with send in Spree::Gateway#method_missing

Change-Id: Iff96153001cb3237b7a6cfe4bec3c25e086ec784

Fixes #2872
Bump to selenium-webdriver 2.32.0
[api] Don't set taxon.parent_id in Api::TaxonsController if parameters contain parent_id

Fixes #2757
Recalculate product count_on_hand when ANY of its variants are saved

Fixes #2799