specify Shakapacker as top-level module

Author: Judahmeek

spec/react_on_rails/packer_utils_spec.rb

@@ -44,7 +44,7 @@ module ReactOnRails
         before do
           allow(described_class.packer).to receive(:dev_server).and_return(
             instance_double(
-              Shakapacker::DevServer,
+              ::Shakapacker::DevServer,
               running?: true,
               protocol: "http",
               host_with_port: "localhost:3035"