skpm publish appcast is reading local and remote
Hello

I am running `skpm publish minor` and I am getting this error

```
t.brasington$ skpm publish minor 
⠋ Bumping package.json version and creating git tag
✖ ERROR Error: ENOENT: no such file or directory, open '/Users/t.brasington/Sites/sketch/text-to-styles/https:/raw.githubusercontent.com/tbrasington/text-to-styles/master/.appcast.xml'
Error: ENOENT: no such file or directory, open '/Users/t.brasington/Sites/sketch/text-to-styles/https:/raw.githubusercontent.com/tbrasington/text-to-styles/master/.appcast.xml'

```

Not sure what has happened since my last publish that is causing it to look in my local and remote source of the appcast file. 

Do i need to change my manifest or package setup to something different? My repo is here https://github.com/tbrasington/text-to-styles

Thank you in advance!

