A few years ago, only a few saisons were widely available in Massachusetts liquor stores: mainly Saison Dupont, brewed in Belgium, and Hennepin, made by Brewery Ommegang in Cooperstown, N.Y. And that was only in stores that stocked solid selections of craft beers. Then came Jack D’Or, the first beer brewed by Cambridge-based Pretty Things Beer & Ale Project. That saison quickly became popular in Boston and then spread elsewhere. This year, interest in saisons exploded. At least half a dozen Massachusetts breweries began making saisons in 2011. Even the venerable Boston Beer Co. put a saison in its summer 12-pack.