Added Travis target for g++ 6.

.travis.yml

@@ -1,21 +1,28 @@
 # Use container-based distribution
 sudo: false
 language: c++
-matrix:
-  include:
-    - compiler: gcc
-      addons:
+addons:
   apt:
     sources:
       - ubuntu-toolchain-r-test
       - boost-latest
     packages:
-            - g++-5
       - libboost-program-options1.55-dev
       - libboost-iostreams1.55-dev
       - libboost-system1.55-dev
       - libboost-filesystem1.55-dev
-      env: COMPILER=g++-5
+matrix:
+  include:
+    - env: COMPILER=g++-5
+      addons:
+        apt:
+          packages:
+            - g++-5
+    - env: COMPILER=g++-6
+      addons:
+        apt:
+          packages:
+            - g++-6
 script:
   - git submodule init
   - git submodule update