Also use g++-4.8

.travis.yml

 
 before_script:
     - if [ "$CC" = "gcc" ]; then export CC="gcc-4.8"; fi
+    - if [ "$CXX" = "g++" ]; then export CXX="g++-4.8"; fi
 
 script:
     - (mkdir build && cd build && cmake .. && make)