Fix engine issues and specs.

- Fix assets so feature specs can run.
- Refactor test env
- Fix 3 of 4 failing specs (the one left id for regression test for
spree/spree#1687, probably something with rails4 sessions). The other
specs was fixed by bypassing gateway error on checkout.
- Refactor syntax for all specs —now they also readable :)
- Minor fixes to the engine, noticed issues with devise that needed to
be loaded before devise-encryptable.
- Bumped devise and cancan + all dev dependencies.
Fix locales

Add missing keys and removed obvious duplications of spree_i18n. Needed
to re order to see what was missing. Actually almost everything is
duplication of spree_i18n except:
- errors.messages.email_is_invalid
- devise.user_passwords.spree_user.*