Old versions of Darwin Core documents exist, but haven't been located

[baskaufs]

As of 2018-04-27, only the metadata for the most recent versions of Darwin Core documents is included in the dataset. However, earlier versions are referenced. For example, the current version of the text guide replaces http://rs.tdwg.org/dwc/2009-12-07/terms/guides/text/, but that URI doesn't dereference. I suppose the old versions are somewhere in the DwC github repo, but I haven't looked for them yet. @tucotuco @peterdesmet