[tor-bugs] #27859 [Core Tor/Tor]: update travis CI to ubuntu xenial image when available

[Tor Bug Tracker & Wiki]

#27859: update travis CI to ubuntu xenial image when available
------------------------+------------------------------
 Reporter:  cyberpunks  |          Owner:  (none)
     Type:  defect      |         Status:  new
 Priority:  Medium      |      Component:  Core Tor/Tor
  Version:              |       Severity:  Normal
 Keywords:  tor-ci      |  Actual Points:
Parent ID:  #27858      |         Points:
 Reviewer:              |        Sponsor:
------------------------+------------------------------
 trusty support ends April 2019, and xenial has
 [https://packages.ubuntu.com/source/xenial-updates/libzstd zstd]

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/27859>
Tor Bug Tracker & Wiki <https://trac.torproject.org/>
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