PepsiCo is betting big on Americans’ love affair with seltzer. Six months after launching its own sparkling water brand, Bubly, the cola giant announced Monday that it is buying SodaStream International, the Israeli company behind the do-it-yourself soda-maker, for $3.2 billion. Seltzer sales have grown rapidly in recent years, as brands like La Croix, Topo Chico and Polar become household names.