Add rake task for re-indexing specialist sectors

This is for https://www.pivotaltracker.com/story/show/77614348

Due to some now-fixed bugs, there are some specialist sectors which are
not correctly indexed in search.  Add a script to re-index these, which
we'll run on production to fix this issue.
Merge pull request #202 from alphagov/script-for-reindexing-specialist-sectors

Add rake task for re-indexing specialist sectors