Fixed config variable in UpdateCheck
Fixed config variable in UpdateCheck

refs https://github.com/TryGhost/Ghost/commit/2e7d0a4e26882c16baaf58f37b932173c80b21e8

- the referenced commit pushed some refactors to the service but
  `this.config` should have just been `config`
- Ghost was 500ing so this commit fixes the incorrect variable

ErisDS/Ghost,e10/Ghost,ErisDS/Ghost,janvt/Ghost,TryGhost/Ghost,janvt/Ghost,e10/Ghost,TryGhost/Ghost,TryGhost/Ghost,ErisDS/Ghost