Update install instruction in README.md

[davidpanofsky]

Changes proposed in this pull request:

• Documentation update because of brew changes:

Warning: Calling brew cask install is deprecated! Use brew install [--cask] instead.

[CLAassistant]

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[acao]

thank you @davidpanofsky !

[brianwarner]

FYI, you can ignore the CLAassistant comment. I've disabled it for this repo since we use EasyCLA now.