[xmlunit/xmlunit.net] ff8278: upgrade nuget in appveyor

[GitHub <no...@gi...>]

  Branch: refs/heads/master
  Home:   https://github.com/xmlunit/xmlunit.net
  Commit: ff8278c171d3fc403a31b8f0db01f14db56d423a
      https://github.com/xmlunit/xmlunit.net/commit/ff8278c171d3fc403a31b8f0db01f14db56d423a
  Author: Stefan Bodewig <ste...@fr...>
  Date:   2019-01-02 (Wed, 02 Jan 2019)

  Changed paths:
    M appveyor.yml

  Log Message:
  -----------
  upgrade nuget in appveyor



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