[EA] Update publications TNA timestamp

The timestamp we currently have is much older, this new one will:
* have PDF links which end up at the cdn.environment-agency.gov.uk URLs which
  are archived by TNA, rather than the rackcdn.com URLs which are not archived
  by TNA.
* have coverage of documents published between the two dates
Merge pull request #602 from alphagov/update_ea_publications_tna_timestamp

[EA] Update publications TNA timestamp