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