Fix missing add-zsh-hook

In older version of zsh, `add-zsh-hook` is not available, causing issues like:

```
/Users/hugo/.oh-my-zsh/lib/termsupport.zsh:32: add-zsh-hook: function definition file not found
/Users/hugo/.oh-my-zsh/lib/termsupport.zsh:33: add-zsh-hook: function definition file not found
```

See https://github.com/robbyrussell/oh-my-zsh/issues/748. This patch pulls in the changes suggested in https://github.com/robbyrussell/oh-my-zsh/issues/748#issuecomment-37862691 by @mcornella and I have tested it on RHEL 5.