Only try to update workflows with a package.json

[cameronsstone]

Adding this check because I use symlinks to link to workflows I'm developing (which are not Node-based). Without this check I get errors similar that reported in #6:

ENOENT: no such file or directory, open '~/Library/Application Support/Alfred/Alfred.alfredpreferences/workflows/my- workflow/package.json'

(However, I still get the "Unable to start Alfred 4" dialog so the code to kill the current alfred doesn't seem to work for me)