use instance hooks provided by hooks instead of changing the gem’s source

middleman-core/lib/middleman-core/application.rb

@@ -33,6 +33,7 @@ module Middleman
 
     # Uses callbacks
     include Hooks
+    include Hooks::InstanceHooks
 
     # Before request hook
     define_hook :before
@@ -240,5 +241,11 @@ module Middleman
     end
     alias :inspect :to_s # Ruby 2.0 calls inspect for NoMethodError instead of to_s
 
+    # Hooks clones _hooks from the class to the instance.
+    # https://github.com/apotonick/hooks/blob/master/lib/hooks/instance_hooks.rb#L10
+    # Middleman expects the same list of hooks for class and instance hooks:
+    def _hooks
+      self.class._hooks
+    end
   end
 end