Update mongo-express to 0.44.0 and node:8-slim

See https://github.com/mongo-express/mongo-express-docker/pull/7

library/mongo-express

@@ -1,5 +1,5 @@
-# maintainer: Nick Cox <[email protected]> (@knickers)
+Maintainers: Nick Cox <[email protected]> (@knickers)
 
-0.42.2: git://github.com/mongo-express/mongo-express-docker@c65871c94790ca0446ec222c5961a2d01c701cc0
-0.42: git://github.com/mongo-express/mongo-express-docker@c65871c94790ca0446ec222c5961a2d01c701cc0
-latest: git://github.com/mongo-express/mongo-express-docker@c65871c94790ca0446ec222c5961a2d01c701cc0
+Tags: 0.44.0, 0.44, latest
+GitRepo: https://github.com/mongo-express/mongo-express-docker.git
+GitCommit: e739e94d5fefd75e5eaf0ff5a6221a45da0d6959