travis: Bump ubuntu version to 20.04
travis: Bump ubuntu version to 20.04

vitasdk/vita-headers,Rinnegatamante/vita-headers,Rinnegatamante/vita-headers,vitasdk/vita-headers,Rinnegatamante/vita-headers,Rinnegatamante/vita-headers,vitasdk/vita-headers,vitasdk/vita-headers