BOSTON — The mastermind of the nationwide college admissions bribery scheme that ensnared celebrities, prominent businesspeople and other parents who used their wealth and privilege to buy their kids’ way into top-tier schools was sentenced to 3 1/2 years in prison Wednesday.The punishment for Rick Singer, 62, is the longest sentence handed down in the sprawling scandal that embarrassed some of the nation’s most prestigious universities and put a spotlight on the secretive admissions system already seen …