(refactor/ci): use xvfb-run instead of manual xvfb setup

- per Travis CI's instructions
  - https://docs.travis-ci.com/user/gui-and-headless-browsers/#using-the-xvfb-run-wrapper
  - also apt addons doesn't seem to be needed
    - speeds up CI a bit too bc no apt-get update

Author: agilgur5

.travis.yml

@@ -2,15 +2,5 @@ language: node_js
 # default is apparently 0.10.48 for some reason O.O
 node_js: '10.16.0'
 
-# add xvfb per Electron CI instructions (https://github.com/electron/electron/blob/master/docs/tutorial/testing-on-headless-ci.md)
-addons:
-  apt:
-    packages:
-      - xvfb
-
-install:
-  # configure xvfb per Electron CI instructions as well
-  - export DISPLAY=':99.0'
-  - Xvfb :99 -screen 0 1024x768x24 > /dev/null 2>&1 &
-  # default install command
-  - npm ci
+# run with xvfb bc it's needed for Electron (https://docs.travis-ci.com/user/gui-and-headless-browsers/#using-the-xvfb-run-wrapper)
+script: xvfb-run npm test