Include duplicate_of in need search results

Without the `duplicate_of` field Maslow’s search results table misrepresents duplicate needs by showing an empty “Decision made” column.

Fixes https://www.pivotaltracker.com/story/show/78193142

Before:
![screen shot 2014-10-03 at 14 17 19](https://cloud.githubusercontent.com/assets/319055/4506281/08190a0c-4b01-11e4-9654-f8a0ea378592.png)

After:
![screen shot 2014-10-03 at 14 17 12](https://cloud.githubusercontent.com/assets/319055/4506285/0e92fbb8-4b01-11e4-912f-a902af318c19.png)
Might be better as `long`, which matches how we send need_ids, although duplicate_of can also be `nil` which I presume is fine.