Add clang++-libc++ to Travis

.travis.yml

@@ -161,6 +161,10 @@ matrix:
             - ubuntu-toolchain-r-test
             - llvm-toolchain-trusty-5.0
 
+    - os: linux
+      compiler: clang++-libc++
+      env: TOOLSET=clang COMPILER=clang++-libc++ CXXSTD=11,14,1z
+
     - os: osx
       compiler: clang++
       env: TOOLSET=clang COMPILER=clang++ CXXSTD=11,14,1z
@@ -182,7 +186,7 @@ install:
 script:
   - |-
     echo "using $TOOLSET : : $COMPILER ;" > ~/user-config.jam
-  - ./b2 -j 3 libs/mp11/test toolset=$TOOLSET cxxstd=$CXXSTD
+  - ./b2 -j3 libs/mp11/test toolset=$TOOLSET cxxstd=$CXXSTD
 
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