WordPress uses it’s own CA bundle, located in WP/wp-includes/certificates.

The CA bundle that was shipped with WordPress up until recently was outdated, as discussed in this issue: https://core.trac.wordpress.org/ticket/45807.

Download an updated version of the bundle, https://github.com/WordPress/WordPress/tree/master/wp-includes/certificates and replace it in the wordpress folder.

Setting sslverify to false is not recommended but you can add this to the Themes function.php

add_filter('https_ssl_verify', '__return_false');