Update Slack oauth URL

[akfaew]

Recently Slack updated its scopes, and the last step of the migration states: "Change your app’s authentication URL to look like this: https://slack.com/oauth/v2/authorize?client_id=XXX"

This URL has changed, and it seems that new Slack apps cannot use this library.

Documentation page: https://api.slack.com/authentication/oauth-v2