use gcc5 in travis build

.travis.yml

@@ -7,11 +7,14 @@ addons:
     sources:
       - ubuntu-toolchain-r-test
     packages:
-      - gcc
-      - g++
+      - gcc-5
+      - g++-5
       - libboost-all-dev
       - cmake
 
+env:
+    - CC=gcc-5 CXX=g++-5
+
 before_script:
   - mkdir build
   - cd build