wrong compiler in linux build

Former-commit-id: 2a7b0b936885ffb38bc4ea687670acc3cb9c52e8

.travis.yml

@@ -11,7 +11,7 @@ matrix:
         - cd python
         - mkdir build
         - cd build
-        - cmake ../
+        - cmake -DCMAKE_CXX_COMPILER=g++-4.8 ../
         - make -j 2
         - cd ../../
         - cd tutorial