NEW YORK — A Rikers Island detainee died of unknown causes following a medical crisis Friday night, marking the jail complex’s second death this year, said a spokesman for the city Correction Department. Manuel Luna, 30, was found unconscious on a bed frame in his cell at Rikers’ George R. Vierno Center at 8:47 p.m. on Friday by a correction captain who was bringing him a mattress, sources told The New York Daily News. The captain immediately began chest compressions in an attempt to revive Luna, the sources said. Medical personnel arrived within a few minutes and tried to resuscitate Luna by …