Install HomeBrew command needs updating

[haitzlm]

Documentation says:

Install Homebrew

Command: /usr/bin/ruby -e "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/master/install)"

Result: Error: The Ruby Homebrew installer is now disabled and has been rewritten in Bash. Please migrate to the following command:
/bin/bash -c "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/HEAD/install.sh)"

Note that homebrew website says same:
/bin/bash -c "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/HEAD/install.sh)"

[hortongn]

@lisa3711 Thanks for this. Feel free to edit rails_development_setup.md with the fix and commit the change. You can do the edit via the github GUI if you want.