WIP, fixes all build jobs except one: 2.7_with_system_site_packages Xenial.

Changes proposed in this pull request:

• Only start Xvfb for Qt, and Qt is only available with the system site packages installs

Yesterday Travis CI released a new build image for Xenial:

https://changelog.travis-ci.com/ubuntu-xenial-16-04-build-environment-update-86123

Unfortunately that broke our Xenial builds. Trusty still works.

• Last master pass: https://travis-ci.org/python-pillow/Pillow/builds/479135721
• First master error: https://travis-ci.org/python-pillow/Pillow/builds/479763270

The Travis changelog says:

The Xenial build environment now comes with PhamtomJS2, a new way to define headless browser testing, via services: xvfb, and improved Java language support.

https://changelog.travis-ci.com/ubuntu-xenial-16-04-build-environment-update-86123

The update history says:

Xvfb init file is removed in favor of a systemd unit file. Xvfb can be set up with services: xvfb. The matching DISPLAY environment variable is made available when you do that.

https://docs.travis-ci.com/user/build-environment-updates/2019-01-14/

We need to find a way to start Xvfb on Xenial.

I tried with these but it didn't help [1] [2]:

    - python:  "2.7_with_system_site_packages" # For PyQt4
      name: "2.7_with_system_site_packages Xenial"
+      services: xvfb

services:
  - docker
+  - xvfb