Fixed bugs that had broken cucumber tests since last spree update

All tests are passing now. The problems were:
- Spree 2.x changed the page rendered after the "update" action in the admin option-type edit page and that broke cucumber tests. I have updated the tests to expect the new behavior;
- The configuration SpreeVariantOptions::VariantConfig.default_instock was being set to "true" while tests expected "false". Now it's set to "false" in the test_helper.rb;
- There was a bug in the javascript handling the variant selection on product page, where it would ignore variants backorder option and disable the add to cart button.