Added Travis target for g++ 6.

.travis.yml

@@ -1,25 +1,42 @@
 # Use container-based distribution
 sudo: false
 language: c++
-matrix:
-  include:
-    - compiler: gcc
 addons:
   apt:
-          sources:
+    sources: &default_sources
       - ubuntu-toolchain-r-test
       - boost-latest
-          packages:
+    packages: &default_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_NAME=g++ _CXX=g++-5
+      os: linux
+      language: cpp
+      addons:
+        apt:
+          sources:
+            - *default_sources
+          packages:
+            - *default_packages
+            - g++-5
+    - env: COMPILER_NAME=g++ _CXX=g++-6
+      os: linux
+      language: cpp
+      addons:
+        apt:
+          sources:
+            - *default_sources
+          packages:
+            - *default_packages
+            - g++-6
 script:
   - git submodule init
   - git submodule update
   - mkdir build
   - cd build
-  - cmake .. -DCMAKE_BUILD_TYPE=Debug -DBUILD_TESTS=ON -DCMAKE_CXX_COMPILER=$COMPILER
+  - cmake .. -DCMAKE_BUILD_TYPE=Debug -DBUILD_TESTS=ON -DCMAKE_CXX_COMPILER=$_CXX
   - make -j3 && make -j3 run-tests