switch Travis-CI build environment to Ubuntu 16.04 ("xenial")

The standard support for Ubuntu 14.04 ("Trusty Tahr") has ended
on 25th of April, 2019. Therefore we should switch to the newer
distribution. The switch also brings a newer GCC.

Author: striezel

.travis.yml

@@ -2,23 +2,18 @@ language: cpp
 compiler:
   - clang
   - gcc
+dist: xenial
 sudo: false
 addons:
   apt:
-    sources:
-    - ubuntu-toolchain-r-test
     packages:
     - cmake
-    - g++-4.8
-    - gcc-4.8
     - freeglut3-dev
     - libarchive-dev
     - libglfw-dev
     - libxml2-dev
     - libzip-dev
     - zlib1g-dev
-before_install:
-  - if [ "$CXX" = "g++" ]; then export CXX="g++-4.8" CC="gcc-4.8"; fi
 script:
   - ./ci/cpplint.sh
   - mkdir ./build