build with multiple gcc versions

.travis.yml

@@ -1,13 +1,16 @@
 language: cpp
 
-compiler: gcc
+env:
+  matrix:
+    - CXX=g++-4.9
+    - CXX=g++-4.8
+    - CXX=g++-4.7
 
 before_install:
  - sudo add-apt-repository ppa:ubuntu-toolchain-r/test -y
  - sudo add-apt-repository ppa:boost-latest/ppa -y
  - sudo apt-get update -qq
- - sudo apt-get install -qq cmake libboost1.54-dev libboost-program-options1.54-dev g++-4.9
- - sudo update-alternatives --install /usr/bin/g++ g++ /usr/bin/g++-4.9 90
+ - sudo apt-get install -qq cmake libboost1.54-dev libboost-program-options1.54-dev ${CXX}
 
 before_script:
  - mkdir build