Upgrade to wp cli 0.15.1

 - Uses published wp-cli.phar from https://github.com/wp-cli/builds/raw/1d252eadc353da8e165f49e8f638e75e694ad92c/phar/wp-cli.phar
 - replaces old ~/.wp-cli/.wp-cli.ini settings with new ~/.wp-cli/config.yml settings