Added doxygen to Travis build chain

Author: Amunak

.travis.yml

@@ -3,8 +3,9 @@ language: cpp
 before_install:
   - sudo add-apt-repository -y ppa:ubuntu-toolchain-r/test
   - sudo apt-get update -qq
-  
+
 install:
+  - sudo apt-get install doxygen
   - sudo apt-get install -qq gcc-4.8 g++-4.8 
   - sudo update-alternatives --install /usr/bin/g++ g++ /usr/bin/g++-4.8 90