move Money::Money outside of spree namespace

Was receiving these warning messages:

```ruby
/Users/benmorgan/.rvm/gems/ruby-2.1.4/bundler/gems/spree_currency_converter-f6489671bdf7/app/models/spree/currency_rate.rb:19: warning: toplevel constant Money referenced by Spree::Money::Money
/Users/benmorgan/.rvm/gems/ruby-2.1.4/bundler/gems/spree_currency_converter-f6489671bdf7/app/models/spree/currency_rate.rb:20: warning: toplevel constant Money referenced by Spree::Money::Money
```

This should fix that.