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.

spec/models/verification/letter_spec.rb

@@ -23,7 +23,7 @@ describe Verification::Letter do
 
     it "updates letter_requested" do
       letter = build(:verification_letter)
-      letter.save
+      letter.save!
       expect(letter.user.letter_requested_at).to be
     end