Unitil Corp. has filed a plan with the state to raise distribution rates that would result in an average monthly electric bill increase of $4.88 and gas bill increase of $13.68, the company announced Tuesday. Unitil, the gas and electricity distributor for Fitchburg, Lunenburg, Townsend, Ashby and a couple of dozen customers in Leominster and Shirley, filed the rate request with the state's Department of Public Utilities.