Add sass-rails to Gemfile.

This is a dependency of spree which is not properly required. The rake
test_app would not run correctly without this missing dependency.
Bump to spree 2-3-stable.

This fixes an issue with rspec 3 + authorization.

https://github.com/spree/spree/pull/5182
Compare state directly in specs.

Previously this test used the invalid? method definition from state
machine which conflicted with a method from ActiveModel. Instead of
dealing with that issue, we will compare the state directly to what it
should be rather than relying upon the magic methods defined by
state_machine.

Fixes #5