Update macOS installation instructions

Fixes #118

src/intro/install/macos.md

@@ -6,16 +6,16 @@ All the tools can be install using [Homebrew]:
 
 ``` console
 $ # GDB
-$ brew cask install gcc-arm-embedded
+$ brew tap armmbed/formulae
+$ brew install armmbed/formulae/arm-none-eabi-gcc
 
+$ # OpenOCD
 $ brew install openocd
 
+$ # QEMU
 $ brew install qemu
 ```
 
-If the `brew cask` command doesn't work (e.g. `error: unknown command: cask`),
-then first run `brew tap Caskroom/tap` and try again.
-
 That's all! Go to the [next section].
 
 [next section]: verify.md