Fix #VALUE! sometimes displaying in the table of transitioning orgs

- This is the same way of fixing it as jobs/org_status.rb.
- We can refactor the two pieces of very similar code into one on a
  separate branch and in a separate pull request.
Refactor the data retrieval method into its own class

- Use this class in org_status.rb & org_top_ten.rb to avoid repetition.
Merge pull request #7 from alphagov/fix_na_value_appearing_in_org_table

Fix #VALUE! sometimes displaying in the table of transitioning orgs