Lock memcached server to version 1.4

Version 1.5 causes problems with test_multi_use_compatibility()

Reported upstream as https://tracker.moodle.org/browse/MDL-60037

Fixes #36

phpunit-external-services.yml

@@ -4,9 +4,9 @@ services:
       environment:
         MOODLE_DOCKER_PHPUNIT_EXTRAS: "true"
   memcached0:
-    image: memcached:1
+    image: memcached:1.4
   memcached1:
-    image: memcached:1
+    image: memcached:1.4
   redis:
     image: redis:3
   solr:
@@ -16,4 +16,4 @@ services:
       - solr-precreate
       - test
   ldap:
-    image: larrycai/openldap
+    image: larrycai/openldap