[TEST] Use `dfs_query_then_fetch` to prevent sporadic failures on BoostingQueriesIntegrationTest

Without this patch, the tests consistently fail couple of times per batch of 15 runs.

    rm -f results.txt
    time for i in {1..15}
      do
        echo "-------------------------------------------------------------------------------"
        echo "TEST $i"
        echo "-------------------------------------------------------------------------------"
        ruby -I lib:test test/integration/boosting_queries_test.rb
      if [ $? == 0 ]; then echo "$i: OK" >> results.txt; else echo "$i: FAIL">> results.txt; fi
    done

    echo; echo; echo "==============================================================================="
    cat results.txt

See <http://www.elasticsearch.org/guide/reference/api/search/search-type/>

Closes #721