Upgrade Ubuntu version in Travis

[ErikBjare]

We're currently using 14.04, which reached end of standard support in 2018-04. We should switch to either 16.04 (Xenial) or 18.04 (Bionic).

Might even solve some hard-to-debug bugs like ActivityWatch/aw-qt#29 and our maybe outdated fontconfig stuff (ActivityWatch/aw-qt#34).

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[johan-bjareholt]

@ErikBjare can we close this? GitHub actions works for the main repo right?

[ErikBjare]

The PyInstaller bundles produced in GitHub Actions don't yet work (see the issue about moving to GitHub Actions).

This issue should maybe still be closed though, as we should just fix the issues in GitHub Actions.

[ErikBjare]

Closing since we're moving to GitHub Actions, so efforts are better spent there.