removed GCC 6, added Clang 3.5

.travis.yml

@@ -30,13 +30,21 @@ matrix:
           packages: ['g++-5', 'valgrind']
       env: COMPILER=g++-5
 
+#    - os: linux
+#      compiler: gcc
+#      addons:
+#        apt:
+#          sources: ['ubuntu-toolchain-r-test']
+#          packages: ['g++-6', 'valgrind']
+#      env: COMPILER=g++-6
+
     - os: linux
-      compiler: gcc
+      compiler: clang
       addons:
         apt:
-          sources: ['ubuntu-toolchain-r-test']
-          packages: ['g++-6', 'valgrind']
-      env: COMPILER=g++-6
+          sources: ['ubuntu-toolchain-r-test', 'llvm-toolchain-precise-3.5']
+          packages: ['clang-3.5', 'valgrind']
+      env: COMPILER=clang++-3.5
 
     - os: linux
       compiler: clang