Error al instalar Homebrew en Mac OS X (10.10.3): No se puede cambiar el modo de archivo en /usr/local/

Cuando ejecuto el comando en la página web de home brew,

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

Lo entiendo:

==> This script will install:
/usr/local/bin/brew
/usr/local/Library/...
/usr/local/share/doc/homebrew
/usr/local/share/man/man1/brew.1
/usr/local/share/zsh/site-functions/_brew
/usr/local/etc/bash_completion.d/brew
==> The following directories will be made group writable:
/usr/local/.
==> The following directories will have their owner set to 17oei:
/usr/local/.
==> The following directories will have their group set to admin:
/usr/local/.

Press RETURN to continue or any other key to abort
==> /usr/bin/sudo /bin/chmod g+rwx /usr/local/.
chmod: Unable to change file mode on /usr/local/.: Operation not permitted
Failed during: /usr/bin/sudo /bin/chmod g+rwx /usr/local/.

Sin embargo, no veo ninguna razón para ello, ya que soy un administrador en mi ordenador.