Travis CI: Do not hardcode trusty because it is EOL

[cclauss]

• Motivation for features / changes
  Travis CI has switch to Xenial as the default distro because Trusty reached the end of its five-year LTS window in April 2019.

• Technical description of changes

Remove the hardcoding of the Trusty distro.

• Screenshots of UI changes

• Detailed steps to verify changes work correctly (as executed by you)

Execute test steps locally and on a branch in Travis CI.

• Alternate designs / implementations considered

Also see #2292

[wchargin]

For reference, I had previously tried to switch over to Xenial in #2293,
but didn’t figure out what was causing CI to fail in the time that I
budgeted.

[wchargin]

Unassigning myself from review because this PR currently introduces
consistent CI failures. I’d love to review it once those are fixed;
please ping me at that time.

@cclauss, it looks like you’re in Switzerland, which is convenient—if
you could do us a favor and minimize the number of CI builds started
from 16:00 to midnight (UTC+2), that’d be much appreciated! Our CI
resources are running a bit thin right now. You’ve been fine so far. :-)

[wchargin]

Resolved by #3070, c/o @caisq. Thanks for the contribution, anyway! :-)