Install-powershell.sh on macOS can install very old version of powershell

in investigating #4471, I found that `install-powershell.sh` was installing an alpha version of PowerShell because it was not updating brew before installing.
Examples:
https://travis-ci.org/richardszalay/pshosts/jobs/260027319#L94
https://travis-ci.org/PowerShell/PowerShell/jobs/261189298#L98