Add call to composer update in wp-cli section, to make sure any new dependencies are added

I just ran into this error message that was caused by a dependency being added to wp-cli, but not being loaded to my preexisting vvv install.

    PHP Fatal error:  Call to undefined function array_column() in /srv/www/wp-cli/php/WP_CLI/SynopsisValidator.php on line 17
Merge pull request #115 from cmmarslender/patch-1

Add call to composer update in wp-cli section, to make sure any new depe...