Bake pie crust according to package directions. Let cool. Toss sliced bananas with lemon juice and spread bananas in the bottom of the baked pie shell. In a pot on top of the stove, mix milk, salt, egg yolks, cornstarch, sugar, vanilla with a spoon or whisk. Add margarine in one piece. Cook on medium high heat about 5 to 7 minutes stirring constantly with whisk. This will burn easily. Mixture will thicken like pudding. Remove from heat and pour over sliced bananas. Let cool before adding meringue.