[macOS] ruby installer for homebrew deprecated

The interactive walkthrough seems to contain instructions to run the installer for homebrew like this:

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

It has been reported in the gitter, that this command exits with a deprecation message.

> The Ruby Homebrew installer is now deprecated and has been rewritten in
Bash.

According to the brew.sh website the new installation command should be

```sh
/bin/bash -c "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/master/install.sh)"
```

It needs to be checked if the current screenshots in the guide are still valid after changing the command.

I still have the opinion, that we should just link to the brew installation instructions, rather than having the step-by-step instructions in the guide.