(back13) Take each multiple of 13 that appears in the array and change all of
the numbers before it to be that multiple of 13, until you come to
another multiple of 13.
Examples:
back13([11,12,13,14]) => [13,13,13,14]back13([40,39,50,52,60]) => [39,39,52,52,60]back13([21,23,25,26,90,65,17]) => [26,26,26,26,65,65,17]