Remove --disable-gpu flag when starting headless chrome
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](https://github.com/ember-cli/ember-cli/pull/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](https://github.com/emberjs/data/pull/6298) but Ember Data's complete
test suite cannot successfully run until all of our external integration
partners have also removed this flag.
ef4/ember-data-relationship-tracker,ef4/ember-data-relationship-tracker