Move require rotp library to the file where it is used

lib/two_factor_authentication.rb

@@ -5,7 +5,6 @@ require "active_model"
 require "active_record"
 require "active_support/core_ext/class/attribute_accessors"
 require "cgi"
-require "rotp"
 
 module Devise
   mattr_accessor :max_login_attempts

lib/two_factor_authentication/models/two_factor_authenticatable.rb

@@ -1,4 +1,6 @@
 require 'two_factor_authentication/hooks/two_factor_authenticatable'
+require 'rotp'
+
 module Devise
   module Models
     module TwoFactorAuthenticatable