Update memcached

Changes:

- https://github.com/docker-library/memcached/commit/3939abc: Update to 1.5.20

library/memcached

@@ -4,12 +4,12 @@ Maintainers: Tianon Gravi <[email protected]> (@tianon),
              Joseph Ferguson <[email protected]> (@yosifkit)
 GitRepo: https://github.com/docker-library/memcached.git
 
-Tags: 1.5.19, 1.5, 1, latest
+Tags: 1.5.20, 1.5, 1, latest
 Architectures: amd64, arm32v5, arm32v7, arm64v8, i386, ppc64le, s390x
-GitCommit: 7b4e8f0188c2f4565f8077c584680df6a93bc263
+GitCommit: 3939abc659b8c72e10484d3771b4b15234b840ca
 Directory: debian
 
-Tags: 1.5.19-alpine, 1.5-alpine, 1-alpine, alpine
+Tags: 1.5.20-alpine, 1.5-alpine, 1-alpine, alpine
 Architectures: amd64, arm32v6, arm32v7, arm64v8, i386, ppc64le, s390x
-GitCommit: 7b4e8f0188c2f4565f8077c584680df6a93bc263
+GitCommit: 3939abc659b8c72e10484d3771b4b15234b840ca
 Directory: alpine