Update memcached stability tag to 2.15.2

Author: wodbot

.env

@@ -112,7 +112,7 @@ ADMINER_TAG=4-3.24.4
 APACHE_TAG=2.4-4.12.1
 ATHENAPDF_TAG=2.16.0
 DRUPAL_NODE_TAG=1.0-2.0.0
-MEMCACHED_TAG=1-2.15.1
+MEMCACHED_TAG=1-2.15.2
 OPENSMTPD_TAG=6-1.18.1
 RSYSLOG_TAG=latest
 SELENIUM_CHROME_TAG=3.141