Restore 'deprecated' function in util.js.

`util.deprecated` is still used in [generators.js](https://github.com/petkaantonov/bluebird/blob/master/src/generators.js#L126). It was improperly removed in 000fc8543a2f57f4df2934ab0bdd3e9d722364e4. Without this patch, use of `Promise.spawn()` will cause the following error:

```
TypeError: undefined is not a function
    at Promise$Spawn (project/node_modules/bluebird/js/main/generators.js:142:5)
   ...
```