Update currency_iso.json – add new ruble sign

On December 11, 2013, the official symbol for the ruble became RUB, a Cyrillic letter Р with a single added horizontal stroke. In Unicode version 7.0 (summer 2014) it was assigned the encoding U+20BD ₽ ruble sign (HTML: &#8381;). by wiki (http://en.wikipedia.org/wiki/Russian_ruble)