travis-ci: change trusty image to xenial

travis-ci: change trusty image to xenial

Ubuntu Trusty reaches End of Life on April 30, 2019
Let us switch to xenial. Also, it simplifies mingw builds.
We do not need to add xenial mingw manually anymore

Signed-off-by: Ilya Shipitsin <chipitsine@gmail.com>
Acked-by: Steffan Karger <steffan.karger@fox-it.com>
Message-Id: <20190311133620.18278-3-chipitsine@gmail.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg18270.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>