Fix one `compiler:` in .travis.yml

.travis.yml

@@ -66,7 +66,7 @@ matrix:
       compiler: g++
 
     - env: SCRIPT=ci_boost_test_library TOOLSET=clang CXXSTD=11
-      compiler: g++
+      compiler: clang++
 
     # Library requirements tests.
     - env: SCRIPT=ci_boost_library_check