Update Whoosh dependency to 2.5.1 or superior. Refs #741.

- Remove the Bloodhound Whoosh workaround/fix (has been fixed in Whoosh 2.5.1)
- Update the tests
  - Whoosh 'score' is now a float instead of a string)
  - Detection of the fix is not necessary anymore
- Update the Whoosh dependency in bloodhound_search setup


git-svn-id: https://svn.apache.org/repos/asf/bloodhound/trunk@1566160 13f79535-47bb-0310-9956-ffa450edef68