Automatically remove non CPython repo installation

[Mariatta]

Blurb_it is meant to be used for CPython only. While it hasn't happened yet, it is quite possible for people to install blurb_it on other repos.

Perhaps blurb_it can be proactive and refuse to be installed on repos other than username/CPython. So if someone tries installing it elsewhere, just remove it.

Using this endpoint, we can remove a repo from installation: https://developer.github.com/v3/apps/installations/#remove-repository-from-installation