travis: move (failing) ubuntu build from xenial to bionic

Ubuntu’s apt started refusing to load package files from unauthenticated
repositories, but the package for which we did that (xcb-xrm) is available in
newer versions of Ubuntu, so I just removed that part altogether.

Apparently this has been broken since April, and nobody noticed :-/

travis/travis-base-ubuntu.Dockerfile

@@ -1,7 +1,7 @@
 # vim:ft=Dockerfile
 # Same as travis-base.Dockerfile, but without the test suite dependencies since
 # we only build Debian packages on Ubuntu, we don’t run the tests.
-FROM ubuntu:xenial
+FROM ubuntu:bionic
 
 RUN echo force-unsafe-io > /etc/dpkg/dpkg.cfg.d/docker-apt-speedup
 # Paper over occasional network flakiness of some mirrors.
@@ -25,10 +25,6 @@ RUN apt-get update && \
 
 # Install i3 build dependencies.
 COPY debian/control /usr/src/i3-debian-packaging/control
-RUN echo 'deb http://dl.bintray.com/i3/i3-autobuild-ubuntu xenial main' > /etc/apt/sources.list.d/i3-autobuild.list && \
-    apt-get update && \
-    apt-get --allow-unauthenticated install i3-autobuild-keyring && \
-    rm -f /var/lib/apt/lists/dl.bintray.com_i3_i3-autobuild-ubuntu_* && \
-    apt-get update && \
+RUN apt-get update && \
     DEBIAN_FRONTEND=noninteractive mk-build-deps --install --remove --tool 'apt-get --no-install-recommends -y' /usr/src/i3-debian-packaging/control && \
     rm -rf /var/lib/apt/lists/*