Update macOS installation - brew cask now supports upgrade

[timothywlewis]

PR Summary

Update macOS installation:
Change "brew cask reinstall powershell" to "brew cask upgrade powershell".

Homebrew/homebrew-cask#29301 was resolved on Nov 27, 2017

PR Checklist

Note: Please mark anything not applicable to this PR NA.

• PR has a meaningful title
  • Use the present tense and imperative mood when describing your changes
• Summarized changes
• [NA] User facing Documentation needed
  • [NA] Issue filed - Issue link:
• [NA] Change is not breaking
• [NA] Make sure you've added a new test if existing tests do not effectively test the code changed
  • [NA] Add [feature] if the change is significant or affects feature tests
• This PR is ready to merge and is not Work in Progress.
  • If the PR is work in progress, please add the prefix WIP: to the beginning of the title and remove the prefix when the PR is ready.

[daxian-dbw]

I have verified that brew cask upgrade powershell works.
Thanks @timothywlewis!