Fix check webpacker version

[xronos-i-am]

Webpacker's version check is failed if webpacker follows for wicked_pdf in Gemfile.

Webpacker is not defined at the gem initialization in the case.

require 'net/http'
# If webpacker is used, need to check for version
require 'webpacker/version' if defined?(Webpacker)

Webpacker::VERSION constant not defined unless 'webpacker/version' required. And webpacker_source_url(source) returns nil

def webpacker_source_url(source)
  return unless defined?(Webpacker) && defined?(Webpacker::VERSION)
  ...
end