In group life policies there shall be a provision that the validity of the policy shall not be contested, except for nonpayment of premiums, after it has been in force for two years from its date of issue; and that no statement made by any person insured under the policy relating to his insurability shall be used in contesting the validity of the insurance with respect to which such statement was made after such insurance has been in force prior to the contest for a period of two years during such person's lifetime nor unless it is contained in a written instrument signed by him