fix background failures

```
Exception
NoMethodError
Error
undefined method `values' for nil:NilClass
/app/app/models/issue.rb:107:in `pr_attached_with_issue?'
/app/app/models/issue.rb:83:in `update_from_github_hash!'
/app/app/models/issue.rb:74:in `find_or_create_from_hash!'
/app/app/models/repo.rb:157:in `block in populate_issue'
/app/app/models/repo.rb:155:in `each'
/app/app/models/repo.rb:155:in `populate_issue'
/app/app/models/repo.rb:166:in `populate_multi_issues!'
/app/app/models/repo.rb:144:in `block in <class:Repo>'
/app/vendor/bundle/ruby/2.1.0/gems/resque_def-0.0.3/lib/resque_def.rb:22:in `call'
/app/vendor/bundle/ruby/2.1.0/gems/resque_def-0.0.3/lib/resque_def.rb:22:in `perform'
```