Remove --disable-gpu flag when starting headless chrome

The `--disable-gpu` flag is [no longer
necessary](https://bugs.chromium.org/p/chromium/issues/detail?id=737678) and, at
least in some cases, is [causing
issues](https://bugs.chromium.org/p/chromium/issues/detail?id=982977).

This flag has already been [removed from ember-cli's
blueprints](ember-cli/ember-cli#8774)

As you may already know, this project's test suite is run as part of [Ember
Data](https://github.com/emberjs/data)'s test suite to help catch regressions.
The flag has already been [removed from Ember Data's own testem
config](emberjs/data#6298) but Ember Data's complete
test suite cannot successfully run until all of our external integration
partners have also removed this flag.

Author: HeroicEric

testem.js

@@ -11,7 +11,6 @@ module.exports = {
       mode: 'ci',
       args: [
         '--headless',
-        '--disable-gpu',
         '--disable-dev-shm-usage',
         '--disable-software-rasterizer',
         '--mute-audio',