travis-ci: add "linux-ppc64le" to build matrix, change trusty image to xenial, update...

travis-ci: add "linux-ppc64le" to build matrix, change trusty image to xenial, update osx to xcode9.4 and modernize brew management

Signed-off-by: Ilya Shipitsin <chipitsine@gmail.com>
Acked-by: Steffan Karger <steffan.karger@fox-it.com>
Message-Id: <20190311133620.18278-2-chipitsine@gmail.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg18271.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit 27fb43da6f86eda7a6e5a033e060cab5be0cdbe9)

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>
(cherry picked from commit 4ca813798af89997441bc35723ed1995b6b269e1)

osx image used for builds, i.e. xcode7.3 is outdated, we
can switch to "default" xcode9.4 and use more fast brew
travis-ci plugin

Signed-off-by: Ilya Shipitsin <chipitsine@gmail.com>
Acked-by: Steffan Karger <steffan.karger@fox-it.com>
Message-Id: <20190311133620.18278-4-chipitsine@gmail.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg18269.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit 2cae16aae55dd1ef2ae41c36bec752db34ab4695)