Travis CI: test upgrading to container-based build

.travis.yml

@@ -1,4 +1,5 @@
 language: cpp
+sudo: false
 
 os:
   - linux
@@ -7,9 +8,13 @@ compiler:
   - gcc
   - clang
 
-before_install:
-  - if [ $TRAVIS_OS_NAME == linux ]; then sudo add-apt-repository -y ppa:pyglfw/pyglfw && sudo apt-get update -qq && sudo apt-get install -y --no-install-recommends libglfw3-dev libxrandr-dev libxi-dev libxxf86vm-dev; fi
-  - if [ $TRAVIS_OS_NAME == osx ]; then brew update && brew install glfw3; fi
+addons:
+  apt:
+    packages:
+      - libglfw3-dev
+      - libxrandr-dev
+      - libxi-dev
+      - libxxf86vm-dev
     
 script: 
   - make -C examples/opengl_example