Refactor currency converter

- Cleaned calculator
- Refactored unneccesary dependency injection in currency_converter
Read [this](https://www.relishapp.com/rspec/rspec-core/v/2-6/docs/helper-methods/let-and-let) and [this](http://stackoverflow.com/a/14118197) and [this](http://stackoverflow.com/a/5359979) about using `let` instead of instance variables in specs.