Use apt to install libglu [doc-build]

.travis.yml

@@ -9,8 +9,6 @@ os:
 services:
   - xvfb
 
-sudo: false
-
 env:
   global:
     - PYENV_VERSION=3.7
@@ -49,6 +47,7 @@ jobs:
       stage: test
       env: DESC="dev test_all"
       before_install:
+        - sudo apt-get install libglu1-mesa
         # install doit/pyctdev and use to install miniconda...
         - pip install pyctdev && doit miniconda_install && pip uninstall -y doit pyctdev
         - export PATH="$HOME/miniconda/bin:$PATH" && hash -r