Remove cxxstd=1z for Clang 5.0

.github/workflows/ci.yml

@@ -84,7 +84,7 @@ jobs:
             install: clang-4.0
           - toolset: clang
             compiler: clang++-5.0
-            cxxstd: "11,14,1z"
+            cxxstd: "11,14"
             container: ubuntu:18.04
             os: ubuntu-latest
             install: clang-5.0