Switch default and alternate symbols for RUB

According to [Wikipedia][1] and [XE][2], the "руб" symbol is more
commonly used. Also added the "₽" symbol as an alternate, because it was
only added to Unicode this year and probably isn't widely supported yet.

[1]: http://en.wikipedia.org/wiki/Russian_ruble#Currency_symbol
[2]: http://www.xe.com/symbols.php