Fix webpacker compatibility

require 'webpacker/version' so that `Webpacker::VERSION` is accessible.
And for consitency, require 'skakapacker/version' when using Shakapacker

Author: rioug

lib/wicked_pdf/wicked_pdf_helper/assets.rb

@@ -232,8 +232,10 @@ def running_in_development?
 
       def webpacker_version
         if defined?(Shakapacker)
+          require 'skakapacker/version'
           Shakapacker::VERSION
         elsif defined?(Webpacker)
+          require 'webpacker/version'
           Webpacker::VERSION
         end
       end