Homebrew installation commands changed

[jhagenberg]

Which Document page:

https://sylabs.io/guides/3.0/user-guide/installation.html#mac

Expected results:

$ brew install --cask virtualbox && \
    brew install --cask vagrant && \
    brew install --cask vagrant-manager

Actual results:

$ brew cask install virtualbox && \
    brew cask install vagrant && \
    brew cask install vagrant-manager

homebrew deprecated cask install and requires install --cask now. It would be great if you could include this change, thank you!