Fix bug in last_otp_at definition

The rotp gem sets this value to an integer, but setting a datetime field to an
integer causes it to be silently set to null.  Make this value an integer instead.

demo/db/migrate/20170516191259_add_lastotpat_to_users.rb

@@ -1,5 +1,5 @@
 class AddLastotpatToUsers < ActiveRecord::Migration
   def change
-    add_column :users, :last_otp_at, :datetime
+    add_column :users, :last_otp_at, :integer
   end
 end

demo/db/schema.rb

@@ -34,6 +34,7 @@
     t.string   "encrypted_otp_secret_salt"
     t.integer  "consumed_timestep"
     t.boolean  "otp_required_for_login"
+    t.integer  "last_otp_at"
   end
 
   add_index "users", ["email"], name: "index_users_on_email", unique: true, using: :btree

lib/devise_two_factor/spec_helpers/two_factor_authenticatable_shared_examples.rb

@@ -119,7 +119,7 @@
         Timecop.travel(subject.otp.interval.seconds.from_now)
       end
 
-      # This currently fails!
+      # This spec tests that reuse of the OTP is not allowed
       it 'fails to validate' do
         expect(subject.validate_and_consume_otp!(consumed_otp)).to be false
       end

lib/generators/devise_two_factor/devise_two_factor_generator.rb

@@ -23,7 +23,7 @@ def create_devise_two_factor_migration
                                 "encrypted_otp_secret_iv:string",
                                 "encrypted_otp_secret_salt:string",
                                 "consumed_timestep:integer",
-                                "last_otp_at:datetime",
+                                "last_otp_at:integer",
                                 "otp_required_for_login:boolean"
                               ]