v39: Record / Update only the major go version

After some issue discussion (heroku/heroku-buildpack-go/100 and another
I can't find atm) I generally think that we should really only care
about the major version of Go and let downstream systems care about
selecting the minor version (the user or build systems). There are
probably exceptions to that, hence I also stopped auto-updating the go
version on save, allowing people to specify whatever they want if they
manually want to manage the file.