switch to installing WordPress via WP-CLI

SVN is on its way out as the WordPress SCM system of choice, and it's
slower than an outright download of latest.zip. WP-CLI will install from
the latest stable version, and doesn't leave a messy trail of .svn
subdirectories.
Caught some wp lines without --allow-root

also since wp-cli.yml specifies the path, no need for the --path option
Merge pull request #7 from DArcMattr/all-wp-cli

Switch to using WP-CLI for installing/updating site files