Use Xenial dist in Travis builds (#50)

.travis.yml

@@ -1,22 +1,14 @@
-sudo: false
+dist: xenial
 language: cpp
 compiler:
   - gcc
   - clang
 addons:
   apt:
-    sources:
-      - george-edison55-precise-backports
-      - ubuntu-toolchain-r-test
     packages:
-      - gcc-4.8
-      - g++-4.8
       - cmake
-      - cmake-data
-      - libboost-dev
       - libboost-program-options-dev
 before_install:
-  - if [ "$CXX" = "g++" ]; then export CXX="g++-4.8" CC="gcc-4.8"; fi
   - export ROOT_TRAVIS_DIR=$(pwd)
   - wget --no-check-certificate https://bitbucket.org/eigen/eigen/get/3.3.2.tar.bz2
   - tar xf 3.3.2.tar.bz2