Fix engine issues + specs + locales.

- [x] Fix assets so feature specs can run #145.
- [x] Refactor test environment.
- [x] Fix 3 of 4 failing specs (the one left is for regression test for spree/spree#1687, probably something with how rails4 sessions works if it passed as expected before). The other specs was fixed by bypassing gateway error on checkout.
- [x] Refactor syntax for all specs —now they also readable :)
- [x] Minor fixes to the engine, noticed issues with `devise` that needed to be loaded before `devise-encryptable` so now you can e.g. generate migrations again without engine errors.
- [x] Bumped devise and cancan + all development dependencies.

@radar Also add locale fixes but add them in separate commit, the remaining are duplication as discussed in  #157 but the question remains where they belong.