Use clang/c++11 instead of 14 because clang 3.9 w/ libstdc++ 4.8 errors on ::gets

.travis.yml

@@ -65,14 +65,14 @@ matrix:
     - env: SCRIPT=ci_boost_status TARGET=quick TOOLSET=clang
       compiler: clang++
 
-    - env: SCRIPT=ci_boost_status TARGET=quick TOOLSET=clang CXXSTD=14
+    - env: SCRIPT=ci_boost_status TARGET=quick TOOLSET=clang CXXSTD=11
       compiler: clang++
 
     # Run tests for the library updated by this commit.
     - env: SCRIPT=ci_boost_test_library TOOLSET=gcc CXXSTD=11
       compiler: g++
 
-    - env: SCRIPT=ci_boost_test_library TOOLSET=clang CXXSTD=14
+    - env: SCRIPT=ci_boost_test_library TOOLSET=clang CXXSTD=11
       compiler: g++
 
     # Library requirements tests.