tfnotify --version not working

## WHAT

I got tfnotify binary from GitHub Releases, But `tfnotify --version` option doesn't seems to work. 

```bash
$ wget https://github.com/mercari/tfnotify/releases/download/v0.7.0/tfnotify_darwin_amd64.tar.gz

$ tar xvf tfnotify_darwin_amd64.tar.gz

$ ./tfnotify --version
tfnotify version unset
```

