Update old 'whatsnew' pages to just link to the versions of those pages in the docs for those versions, which will always be properly rendered for their associated Astropy release.  Left the 0.1 one alone, as there was never really a 'whatsnew' page in Astropy 0.1
Merge pull request #2515 from embray/docs/old-whatsnew

Replace "What's New" page for previous versions with links