That's unfortunately a complex task, though it doesn't look it immediately. How are you defining things in your Puppetfile? Are you pulling from git's master, a git tag or branch, or from the forge? Do you want your fixtures to be using HEAD or the specific version in your Puppetfile? If the latter, how can it correlate between forge versions and git branch/tags? I think that complexity has held up anyone from writing a full featured rake task to keep fixtures up to date, at least anything that I've seen advertised publicly.
There are some tools to update the Puppetfile, though -
https://github.com/puppetlabs/r10k/blob/master/doc/updating-your-puppetfile.mkd. I wrote the last one, generate-puppetfile, which can create a very naive .fixtures.yml for you, assuming you want to test against HEAD. If you want to do specific versions, you'll have to do something else. PRs accepted :)