Only show latest doc collections in speed tagger

Previously all editions of every document collection were being shown
which meant the same document collection would show multiple times with
no indication which should be chosen. This change makes only the latest
edition for each document collection display.
Merge pull request #1066 from alphagov/fix-document-collection-speed-tagging

Only show latest doc collections in speed tagger
Previously all editions of every document collection were being shown
which meant the same document collection would show multiple times with
no indication which should be chosen. This change makes only the latest
edition for each document collection display.

Further fix for https://www.pivotaltracker.com/story/show/59242784