ci: Removing support for Ubuntu 16.04

Ubuntu 16.04 has reached EOL and the 2.6.0 branch does not compile
there.

.travis.yml

@@ -22,10 +22,6 @@ matrix:
       os: linux
       env: CONTAINER=debiantesting
       services: docker
-    - name: "Ubuntu 16.04"
-      os: linux
-      env: CONTAINER=ubuntu1604
-      services: docker
     - name: "Ubuntu 18.04"
       os: linux
       env: CONTAINER=ubuntu1804

test/docker/ubuntu1604

@@ -1,27 +0,0 @@
-FROM ubuntu:16.04
-
-RUN apt-get update
-RUN apt-get install -y build-essential cmake git uuid-dev libgnutls28-dev faketime locales python3
-
-# Setup language environment
-RUN locale-gen en_US.UTF-8
-ENV LC_ALL en_US.UTF-8
-ENV LANG en_US.UTF-8
-ENV LANGUAGE en_US.UTF-8
-
-# Setup taskwarrior
-ADD . /root/code/
-WORKDIR /root/code/
-RUN git clean -dfx
-RUN git submodule init
-RUN git submodule update
-RUN cmake -DCMAKE_BUILD_TYPE=debug .
-RUN make -j2
-RUN make install
-RUN task --version
-
-# Setup tests
-WORKDIR /root/code/test/
-RUN make
-
-CMD ["bash", "-c", "./run_all -v ; cat all.log | grep 'not ok' ; ./problems"]