Add save! method to ActiveModel models

This way we make it clear we expect records to be valid when we save
them, just like we do with ActiveRecord models.

app/controllers/verification/letter_controller.rb

@@ -15,7 +15,7 @@ class Verification::LetterController < ApplicationController
 
   def create
     @letter = Verification::Letter.new(user: current_user)
-    @letter.save
+    @letter.save!
     redirect_to letter_path
   end