Tentative fix of Travis CI

.travis.yml

@@ -30,9 +30,17 @@ matrix:
             - llvm-toolchain-precise-3.6
           packages:
             - clang-3.6
-      env: COMPILER=clang++-3.6
-compiler:
-  - clang
+      env:
+        - MATRIX_EVAL="CC=clang-3.6 && CXX=clang++-3.6"
+    - compiler: clang
+      addons:
+        apt:
+          sources:
+            - llvm-toolchain-trusty-5.0
+          packages:
+            - clang-5.0
+      env:
+        - MATRIX_EVAL="CC=clang-5.0 && CXX=clang++-5.0"
 
 before_script: