Removed rewire dependency in update check tests
Removed rewire dependency in update check tests

refs https://github.com/TryGhost/Team/issues/728

- Rewire is usually a sign of massive module that is impossible to unit test without mocking big parts of that module. With a refactored UpdateCheckService and a brand new test coverage there is no need to use rewire and leave the "smell"

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