Switch to the newer Whitehall mappings CSV

This should be quicker as the newer CSV is smaller: 110k lines vs 190k lines,
and should make it easier to implement tracking pending content.
Merge pull request #128 from alphagov/switch_to_newer_whitehall_mappings_csv

Switch to the newer Whitehall mappings CSV
Merge commit '6a7d2d5ea56019038855aff58cd5984879af56d3' into HEAD