Frontier Airlines is expanding its presence at Philadelphia International Airport starting in June with the launch of its new nonstop service to San Juan, Puerto Rico. The year-round flight will be the 20th nonstop destination out of PHL for Frontier, and offers travelers a "low-cost option," according to Josh Flyr, a Frontier VP. The debut flight to the SJU airport will be the evening of June 11 and it will return to PHL the next morning. “We are very excited that Frontier will be offering…