Build the flatpak in travis.

Use docker to have the last version of ubuntu (and flatpak)

travis/Dockerfile_flatpak

@@ -0,0 +1,63 @@
+FROM ubuntu:bionic
+
+ENV LANG C.UTF-8
+
+RUN \
+  apt update -q; \
+  apt full-upgrade --purge -q -y; \
+  apt install -q -y --no-install-recommends \
+# Base build tools
+    build-essential \
+    automake \
+    libtool \
+    cmake \
+    ccache \
+    pkg-config \
+    autopoint \
+    patch \
+    python \
+    python3 \
+    python3-pip \
+    python3-setuptools \
+    git \
+    subversion \
+    wget \
+    unzip \
+    sudo \
+# Flatpak tools
+    elfutils \
+    flatpak \
+    flatpak-builder \
+# Some helper tools
+    vim \
+    less \
+    grep \
+    openssh-client \
+    ; \
+  apt-get clean -y; \
+  rm -rf \
+    /usr/share/doc/* \
+    /var/cache/debconf/*
+
+RUN useradd --create-home travis -G sudo
+RUN echo '%sudo ALL=(ALL) NOPASSWD:ALL' >> /etc/sudoers
+USER travis
+
+WORKDIR /home/travis
+
+RUN \
+  mkdir -p /home/travis/.local/bin ;\
+  wget https://github.com/ninja-build/ninja/releases/download/v1.8.2/ninja-linux.zip ;\
+  unzip ninja-linux.zip ninja ;\
+  mv ninja /home/travis/.local/bin ;\
+  rm ninja-linux.zip
+ENV PATH="/home/travis/.local/bin:${PATH}"
+
+COPY . kiwix-build/
+RUN sudo chown -R travis:travis /home/travis/kiwix-build
+RUN pip3 install --user -e kiwix-build
+
+ENV TRAVISCI_SSH_KEY /home/travis/kiwix-build/travis/travisci_builder_id_key
+ENV TRAVIS_OS_NAME linux_artful
+
+CMD kiwix-build/travis/compile_all.py && kiwix-build/travis/deploy.sh