Upgrade Ubuntu to latest LTS (Bionic 18.04)

Upgrade Ubuntu to latest LTS (Bionic 18.04), including:
- Change the IMAGE used for base-notebook;
- Keep Mesos install from Mesosphere's Xenial repository, since they
  don't yet have a Bionic repository;
- gnupg needs to be installed for apt-key to work;
- --force-yes option to apt-get has been removed;
- Package libav-tools is replaced by ffmpeg;
- Update Let's Encrypt example script to use 18.04.

pyspark-notebook/Dockerfile

@@ -23,13 +23,15 @@ RUN cd /tmp && \
 RUN cd /usr/local && ln -s spark-${APACHE_SPARK_VERSION}-bin-hadoop${HADOOP_VERSION} spark
 
 # Mesos dependencies
-RUN . /etc/os-release && \
+# Install from the Xenial Mesosphere repository since there does not (yet)
+# exist a Bionic repository and the dependencies seem to be compatible for now.
+RUN apt-get -y update && \
+    apt-get install --no-install-recommends -y gnupg && \
     apt-key adv --keyserver hkp://p80.pool.sks-keyservers.net:80 --recv E56151BF && \
-    DISTRO=$ID && \
-    CODENAME=$VERSION_CODENAME && \
-    echo "deb http://repos.mesosphere.io/${DISTRO} ${CODENAME} main" > /etc/apt/sources.list.d/mesosphere.list && \
+    echo "deb http://repos.mesosphere.io/ubuntu xenial main" > /etc/apt/sources.list.d/mesosphere.list && \
     apt-get -y update && \
-    apt-get --no-install-recommends -y --force-yes install mesos=1.2\* && \
+    apt-get --no-install-recommends -y install mesos=1.2\* && \
+    apt-get purge --auto-remove -y gnupg && \
     apt-get clean && \
     rm -rf /var/lib/apt/lists/*