Sorry, I'm not clear on which launcher you are using. I'll explain in more detail.
When using the new launcher, you can choose any update from 1.7.12 and above.
When using the old launcher, you can only choose from updates you have downloaded previously, or the latest. Since the latest at this time is 1.8.13 and anything above 1.8.00 is incompatible with the old launcher, using the latest will get you the "launcher update required" prompt.
That the old launcher is so limited to what update can be chosen is a big problem. This is why we put other very important things on hold (skinning, updating the runtimes for FFD, etc) and updated the launcher now. I wish it could have been done in a way that was compatible with the older versions, but its actually extremely complicated. There are many edge cases for updates and its quite fragile, if something in the launcher goes wrong then people can't use Spine at all, and that is bad. It took weeks to get the new launcher working.
At any rate, if you would like to use 1.7.12 then its easiest to use the new launcher and put 1.7.12 in the version.txt. I would like to fix the problem you are having though. Maybe we could chat on IRC or Skype and work through it? Maybe we could do Team Viewer or JoinMe and I could investigate directly on a machine with the problem. Currently it's very difficult for me to debug since I can't see it fail.