1.2.0 is the last official, general-distribution build.
The 1.3.0 version has been going through several beta updates. You can change in Steam to request the beta branch, but if you make maps with APIs more recent than 1.2.0, players with the older version may have issues where the maps seem broken.
To use the beta Branch, left-click on the game in your Steam library, select Properties -> Betas and select the beta Branch from there.
Knucracker has indicated that he's keen to release the 1.3.0 update, and that he was simply waiting for bug reports and change requests to the API to reduce and settle for a few days before he release.
'If you want to live a happy life, tie it to a goal, not to people or things.'
Albert Einstein
(1879-1955)