Update memcached

Changes:

- https://github.com/docker-library/memcached/commit/6923e9c: Merge pull request https://github.com/docker-library/memcached/pull/62 from infosiftr/parallel-test
- https://github.com/docker-library/memcached/commit/af2f393: Add "PARALLEL" to "make test" invocation

library/memcached

@@ -6,10 +6,10 @@ GitRepo: https://github.com/docker-library/memcached.git
 
 Tags: 1.6.8, 1.6, 1, latest
 Architectures: amd64, arm32v5, arm32v7, arm64v8, i386, mips64le, ppc64le, s390x
-GitCommit: dcaec15d8dcdf7ae6929b6c81e7bcc7c95a5fc75
+GitCommit: af2f39379f20b6d3f7d31df7295cb85144aa7f51
 Directory: debian
 
 Tags: 1.6.8-alpine, 1.6-alpine, 1-alpine, alpine
 Architectures: amd64, arm32v6, arm32v7, arm64v8, i386, ppc64le, s390x
-GitCommit: dcaec15d8dcdf7ae6929b6c81e7bcc7c95a5fc75
+GitCommit: af2f39379f20b6d3f7d31df7295cb85144aa7f51
 Directory: alpine