travis: stop using the official upstream-systemd-ci repository

Turns out the key for the repository hasn't been propagated properly
so let's restore the kludge that was removed in https://github.com/systemd/systemd/pull/11582.
Of course it's ugly but at least it works.

The issue was kind of reported to the maintainers of the repository
in https://github.com/systemd/systemd/pull/11531#issuecomment-460023474.

travis-ci/managers/xenial.sh

@@ -3,7 +3,6 @@
 set -e
 set -x
 
-add-apt-repository ppa:upstream-systemd-ci/systemd-ci -y
 apt-get update
 apt-get build-dep systemd -y
 apt-get install -y util-linux libmount-dev libblkid-dev liblzma-dev libqrencode-dev libmicrohttpd-dev iptables-dev liblz4-dev libcurl4-gnutls-dev unifont itstool kbd cryptsetup-bin net-tools isc-dhcp-client iputils-ping strace qemu-system-x86 linux-image-virtual mount libgpg-error-dev libxkbcommon-dev python-lxml python3-lxml python3-pip libcap-dev
@@ -12,6 +11,8 @@ pip3 install meson
 
 cd $REPO_ROOT
 
+sed -i 's/2\.30/2.27/' meson.build
+
 meson --werror -Db_sanitize=address,undefined -Dsplit-usr=true build
 ninja -v -C build
 make -C test/TEST-01-BASIC clean setup run TEST_NO_QEMU=yes NSPAWN_ARGUMENTS=--keep-unit RUN_IN_UNPRIVILEGED_CONTAINER=no