Invitation notifications link to no where

[levithomason]

I have received a notification of an invite. It shows up in Gitify, but clicking it does nothing. The notification list in GitHub's web interface does link to the proper place. Here is the notification url that should have been used:

https://github.com/layershifter/babel-plugin-transform-react-handled-props/invitations

[setchy]

Will address shortly after #793