Why does iodine have a higher boiling point than chlorine?

Neither iodine or chlorine have permanent dipoles because there is no difference in electronegativity between the atoms, so there are no permanent dipole : permanent dipole interactions. There are also no hydrogen bonds, so the only intermolecular interactions involved are instantaneous dipole : induced dipole attractions.
Iodine is a larger molecule than chlorine, meaning it has more electrons. Therefore the London forces between neighbouring iodine molecules are stronger than those between neighbouring chlorine molecules. This means that they require more energy to overcome and hence the boiling point is higher.