Added Travis target for g++ 6.

.travis.yml

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