added missing currencies which have exchange rates

Open Exchange Rates had more currencies than this file.
I added all the missing currency configurations.
I figured if there's an active exchange rate, it should exist in currency.json.

```ruby
open_ex = JSON.parse(open('http://openexchangerates.org/api/latest.json?app_id=XYZ').read)['rates']
money_gem = JSON.parse(IO.read('currency.json'))

open_ex.keys - money_gem.keys.collect(&:upcase) 
# => ["BTC", "CLF", "EEK", "JEP", "MTL", "XAG", "XAU", "XDR", "ZMW", "ZWL"]

```