travis: set up xvfb for vggio tests

.travis.yml

@@ -1,5 +1,7 @@
 sudo: true
 
+dist: bionic
+
 language: go
 go_import_path: gonum.org/v1/plot
 
@@ -20,6 +22,10 @@ env:
 
 # Get coverage tools, and start the virtual framebuffer.
 before_install:
+ - sudo apt-get update
+ - sudo apt-get install -qq pkg-config libwayland-dev libx11-dev libx11-xcb-dev libxkbcommon-dev libxkbcommon-x11-dev libgles2-mesa-dev libegl1-mesa-dev libffi-dev libxcursor-dev xvfb xdotool
+ - Xvfb :99 -screen 0 1920x1024x24 &
+
  # Required for format check.
  - go get golang.org/x/tools/cmd/goimports
  # Required for imports check.
@@ -30,9 +36,6 @@ before_install:
  - go get golang.org/x/tools/cmd/cover
  - go get github.com/mattn/goveralls
 
-before_script:
- - sudo apt-get install -qq pkg-config libwayland-dev libx11-dev libx11-xcb-dev libxkbcommon-dev libxkbcommon-x11-dev libgles2-mesa-dev libegl1-mesa-dev libffi-dev libxcursor-dev
-
 # Get deps, build, test, and ensure the code is gofmt'ed.
 # If we are building as gonum, then we have access to the coveralls api key, so we can run coverage as well.
 script: