Install the beta tester plugin and upgrade to the latest nightly build, iPhone 6+

#gallery-91 {
margin: auto;
}
#gallery-91 .gallery-item {
float: left;
margin-top: 10px;
text-align: center;
width: 33%;
}
#gallery-91 img {
border: 2px solid #cfcfcf;
}
#gallery-91 .gallery-caption {
margin-left: 0;
}
/* see gallery_shortcode() in wp-includes/media.php */

#beta-testing-flow, #captions-needed, #ios, #phablet, #plugins, #update, #updates, #upgrade, #upgrade-install, #web