DocumentCollection indexes slug.

Seems like slug was required after all.  Restoring it.
Merge pull request #978 from alphagov/fix-indexing-of-document-collections

DocumentCollection indexes slug.

https://www.pivotaltracker.com/story/show/59311828
Merge commit '1126a327ece697dd004f6b08b9b1b0a7fd0ae2f4' into HEAD