Add um to spree countries

Adding the "United States Minor Outlying Islands" country to the Spree default countries list.

Also fixing the number code for Jersey.
I thought initially it was the international dialling code for the country, but then I double checked and in fact it appears to be the ISO 3166-1 numeric codes (http://en.wikipedia.org/wiki/ISO_3166-1_numeric). 

I'm finding a few more missing countries (Guernsey, Isle of Man) - it might be best to take a systematic approach and take the list from a reputable source and add the full list of missing countries.