Recorrect the solution of Number/credit_card_validator and improve it

Recorrect:
As shown in [http://en.wikipedia.org/wiki/Luhn_algorithm], the calculation site is from right to left, thus, before processing, the input-list should be inversed.
Improve:
y = x*2, x=1,2,3...9; the sum of the digits in y is equal to (y-9), so using the latter is much well.