Use container-based environment on Travis

Now that https://github.com/travis-ci/apt-package-whitelist/pull/2668 is merged, we can specify `sudo: false`.

.travis.yml

@@ -1,16 +1,10 @@
 language: c
-
-# unfortunately gtkgl is not allowed yet as an APT addon
-sudo: required
-dist: trusty
+sudo: false
 
 compiler:
   - clang
   - gcc
 
-before_install:
-  - sudo apt-get install -y libgtkgl2.0-dev
-
 install:
   - ./autogen.sh
   - ./configure --prefix=/tmp/fsv
@@ -27,3 +21,4 @@ addons:
       - libgtk2.0-dev
       - libgl1-mesa-dev
       - libglu1-mesa-dev
+      - libgtkgl2.0-dev