Fix postgres error when displaying network graph.

fix #4149

app/models/network/graph.rb

@@ -25,7 +25,7 @@ module Network
     def collect_notes
       h = Hash.new(0)
       @project.notes.where('noteable_type = ?' ,"Commit").group('notes.commit_id').select('notes.commit_id, count(notes.id) as note_count').each do |item|
-        h[item["commit_id"]] = item["note_count"]
+        h[item.commit_id] = item.note_count.to_i
       end
       h
     end