Add clang-4.0 to Travis

.travis.yml

@@ -189,6 +189,17 @@ matrix:
             - ubuntu-toolchain-r-test
             - llvm-toolchain-precise-3.9
 
+    - os: linux
+      compiler: clang-4.0
+      env: TOOLSET=clang COMPILER=clang++-4.0 CXXSTD=c++11
+      addons:
+        apt:
+          packages:
+            - clang-4.0
+          sources:
+            - ubuntu-toolchain-r-test
+            - llvm-toolchain-trusty-4.0
+
     - os: osx
       env: TOOLSET=clang COMPILER=clang++ CXXSTD=c++11