travis: Bump up doxygen version to 1.8.20
travis: Bump up doxygen version to 1.8.20

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