Try fix Clang builds

Author: d-frey

.travis.yml

@@ -34,12 +34,20 @@ matrix:
           packages: ['g++-6']
       env: COMPILER=g++-6
 
+    - os: linux
+      compiler: clang
+      addons:
+        apt:
+          sources: ['ubuntu-toolchain-r-test']
+          packages: ['g++-4.9']
+      env: COMPILER=clang++
+
     - os: linux
       compiler: clang
       addons:
         apt:
           sources: ['ubuntu-toolchain-r-test', 'llvm-toolchain-precise-3.5']
-          packages: ['clang-3.5']
+          packages: ['clang-3.5', 'g++-4.9']
       env: COMPILER=clang++-3.5
 
     - os: linux