Refactored update check to use async/await syntax
Refactored update check to use async/await syntax

refs https://github.com/TryGhost/Team/issues/726

- These are minimal changes that I've done while reviewing the code inside the update-check module. There's more to come, only picked up the low-hanging fruit!

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