removed Clang 5.0 -std=c++1z as not supported by Mp11

Author: joaquintides

.github/workflows/ci.yml

@@ -92,7 +92,7 @@ jobs:
             install: clang-4.0
           - toolset: clang
             compiler: clang++-5.0
-            cxxstd: "11,14,1z"
+            cxxstd: "11,14"
             os: ubuntu-latest
             container: ubuntu:18.04
             install: clang-5.0