CompatHelper: bump compat for Optim to 2, (keep existing compat)

This pull request changes the compat entry for the `Optim` package from `1.7.5` to `1.7.5, 2`. This keeps the compat entries for earlier versions. Note: I have not tested your package with this new compat entry. It is your responsibility to make sure that your package tests pass before you merge this pull request.