Merge pull request #520 from muupan/fix-ci-with-xenial

Specify ubuntu 16.04 for Travis CI and modify a dependency accordingly

.travis.yml

@@ -1,15 +1,18 @@
 language: python
+dist: xenial
 cache: pip
 python:
   - "2.7"
   - "3.6"
 env:
   - CHAINER_VERSION=4
   - CHAINER_VERSION=stable
+services:
+  - xvfb
 before_install:
-  - sudo add-apt-repository -y ppa:mc3man/trusty-media
+  - sudo add-apt-repository -y ppa:jonathonf/ffmpeg-4
   - sudo apt-get update
-  - sudo apt-get install -y ffmpeg
+  - sudo apt install -y freeglut3-dev ffmpeg
 # command to install dependencies
 install:
   - pip install --upgrade pip setuptools wheel
@@ -39,10 +42,6 @@ install:
   - pip install pybullet
   - python setup.py develop
   - python -c "import numpy; numpy.show_config()"
-before_script:
-  - "export DISPLAY=:99.0"
-  - sh -e /etc/init.d/xvfb start
-  - sleep 3
 # command to run tests
 script:
   - flake8 chainerrl