Require github_api for RepoActivator

Fixes the following error when activating a repo:

> NameError (uninitialized constant RepoActivator::GithubApi):
>   app/services/repo_activator.rb:4:in `block in activate'
>   app/services/repo_activator.rb:22:in `change_repository_state_quietly'
>   app/services/repo_activator.rb:3:in `activate'
>   app/controllers/activations_controller.rb:10:in `create'