Change the hmrc-manuals prefix to hmrc-internal-manuals

- These manuals aren't normal manuals, they're internal manuals
  published solely because of FoI requests. End users should therefore
  be made aware that they're different (and in a different style, etc.)
  hence this change.
Merge pull request #113 from alphagov/change_hmrc_manuals_prefix

Change the hmrc-manuals prefix to hmrc-internal-manuals