Allow Valitor gateway to accept other currencies

This Valitor gateway used to only accept the ISK currency, which is a non-decimal currency.  So we used to always round the amount before sending it on to the gateway.

Now, this gateway accepts other currencies, like USD.  So at this point in time if you purchase through Valitor in USD, your order is always rounded to the nearest dollar.  Like this:

https://www.monosnap.com/image/Hml7dSfv8rims7j01wXmupCmU
https://www.monosnap.com/image/bgFY6N8D892R0J2UOLHxG8CDv

This PR will keep the correct behaviour for the ISK currency, and correctly format the cents if you're paying with a non-decimal currency.  I've also added ISK to the `CURRENCIES_WITHOUT_FRACTIONS` in `Gateway.rb`.

Review
@melari @Soleone 

cc/
@benjlcox