Added tests with GCC 6

.travis.yml

@@ -14,6 +14,14 @@ matrix:
           packages:
             - g++-4.9
       env: COMPILER=g++-4.9
+    - compiler: gcc
+      addons:
+        apt:
+          sources:
+            - ubuntu-toolchain-r-test
+          packages:
+            - g++-6
+      env: COMPILER=g++-6
     - compiler: clang
       addons:
         apt: