homu moved; webhook update required

[Colin Walters]

Homu has been migrated into a more stable home in CentOS CI:
https://homu-projectatomic-ci.apps.ci.centos.org/

But a flatpak repo admin needs to update the flatpak webhooks to point there;
just replace the old  one, but keep the /github suffix.  In other words
the webhook should point to

https://homu-projectatomic-ci.apps.ci.centos.org/github

For a bit more background see 
https://lists.centos.org/pipermail/ci-users/2017-November/000743.html