Change wikiversions to use json

- wikiversions.dat -> wikiversions.json
- Rename switchAllMediaWikis to updateWikiversions
- Remove half-assed $oldVersion validation
- Get rid of populateWikiversions, since it is now served by
  updateWikiversions.

Change-Id: Ic804421a6fffd044d93d180edee974b900820daa
Merge "Change wikiversions to use json"