Fix WP_CLI::colorize warnings

Passing an array to colorize causes warnings since the passed in value ends up being passed
into md5

Warning was
PHP Warning:  md5() expects parameter 1 to be string, array given in git/wp-cli/vendor/wp-cli/php-cli-tools/lib/cli/Colors.php on line 118
Merge pull request #1356 from jeichorn/colorize_warnings

Fix WP_CLI::colorize warnings