missing lua file: model_utils/optim_updates.lua

In model.lua, there is this require:
```lua
require 'model_utils.optim_updates'
```
That file does not exist in this repo. I found it in the visdual repo and created a symbolic link in the base directory of visual-chatbot:

```
ln -s /opt/visdial/model_utils .
```
This could be solved with a git module like neuraltalk2 or by including those directories in this repo.