Merge pull request #5505 from scientificlinux/sl6

Update sl container for sl6 to current (build-date=20190304)

library/sl

@@ -15,4 +15,4 @@ Tags: 6
 Architectures: amd64
 GitFetch: refs/heads/6
 Directory: sl6
-GitCommit: d869ef8987d92b2e4616b79ab64c2497ac46a16f
+GitCommit: b4a7d173c8fd026c79fe2f2cb32f4dd65bbc2c99