use gcc 4.8 with travis

.travis.yml

@@ -3,8 +3,10 @@ language: cpp
 compiler: gcc
 
 before_install:
+ - sudo add-apt-repository ppa:ubuntu-toolchain-r/test -y
  - sudo apt-get update -qq
- - sudo apt-get install -qq cmake libboost-all-dev
+ - sudo apt-get install -qq cmake libboost-all-dev g++-4.8
+ - sudo update-alternatives --install /usr/bin/g++ g++ /usr/bin/g++-4.8 90
 
 before_script:
  - mkdir build