ci: Remove cxxstd=14 from clang 3.5 job on GitHub Actions (#590)

Despite https://clang.llvm.org/cxx_status.html saying:

  Clang 3.4 and later implement all of the ISO C++ 2014 standard.

The clang 3.5 suffers from the bug in C++14 mode causing failure of
Boost.Filesystem build with:

  error: debug information for auto is not yet supported
  error: debug information for auto is not yet supported

https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=800483

.github/workflows/ci.yml

@@ -38,7 +38,7 @@ jobs:
             os: ubuntu-18.04
           - toolset: clang
             compiler: clang++-3.5
-            cxxstd: "11,14"
+            cxxstd: "11"
             define: "_GLIBCXX_USE_CXX11_ABI=0"
             os: ubuntu-16.04
             install: clang-3.5