set package.json path to match Shakapacker requirements

Author: Judahmeek

lib/react_on_rails/version_checker.rb

@@ -74,7 +74,7 @@ def self.build
       end
 
       def self.package_json_path
-        Rails.root.join("client", "package.json")
+        Rails.root.join("package.json")
       end
 
       def initialize(package_json)