Remove Currency in favor of Money::Currency
Remove i18n
Remove Assessor
Move to money-compatible banks

Big refactoring:

- Remove redis and persistence
- Move to [money](https://github.com/RubyMoney/money)-compatible banks
- Get rid of autoloading
Use mutex in #update_rates!
Remove rr dependency
Better dependency versions and gemspec description
Only ignore *.gem in pkg/
Update README
Update README