Fix compatibility with omniauth-oauth2 v1.4+

References:
* omniauth/omniauth-oauth2#70
* omniauth/omniauth-oauth2@2615267
* omniauth/omniauth-oauth2#82
* jdennes/omniauth-createsend#3
* WebTheoryLLC/omniauth-twitch#4

lib/omniauth/strategies/drip.rb

@@ -1,9 +1,8 @@
-require 'omniauth-oauth2'
+require "omniauth/strategies/oauth2"
 
 module OmniAuth
   module Strategies
     class Drip < OmniAuth::Strategies::OAuth2
-
       option :name, 'drip'
 
       option :access_token_options, {
@@ -38,6 +37,10 @@ def user_info
       def raw_info
         @raw_info ||= JSON.parse(access_token.get("/v2/accounts").body)
       end
+
+      def callback_url
+        full_host + script_name + callback_path
+      end
     end
   end
 end