message: Improve composer installer on darwin
  Check to signature.
  Add error handling.
diff: @@ -6,9 +6,23 @@ ini_file=$(php --ini | grep "Loaded Configuration" | sed -e "s|.*:s*||" | sed "s
  ext_dir=$(/usr/bin/php -i | grep "extension_dir => /usr" | sed -e "s|.*=> s*||")
  sudo chmod 777 "$ini_file"
  mkdir -p "$(pecl config-get ext_dir)"
  -curl -sS https://getcomposer.org/installer | php
  -chmod +x composer.phar
  -mv composer.phar /usr/local/bin/composer
  +if [ ! -e "/usr/bin/composer" ]; then
  + EXPECTED_SIGNATURE="$(curl -s https://composer.github.io/installer.sig)"
  + curl -s -L https://getcomposer.org/installer > composer-setup.php
  + ACTUAL_SIGNATURE="$(php -r "echo hash_file('sha384', 'composer-setup.php');")"
  +
  + if [ "$EXPECTED_SIGNATURE" != "$ACTUAL_SIGNATURE" ]; then
  + >&2 echo 'ERROR: Invalid installer signature'
  + rm composer-setup.php
  + exit 1
  + fi
  +
  + COMPOSER_ALLOW_SUPERUSER=1
  + sudo php composer-setup.php --install-dir=/usr/local/bin --filename=composer
  + RESULT=$?
  + rm composer-setup.php
  + exit $RESULT
  +fi
  composer global require hirak/prestissimo
  php -v
  composer -V
  