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.

scipy-notebook/Dockerfile

@@ -6,9 +6,9 @@ LABEL maintainer="Jupyter Project <jupyter@googlegroups.com>"
 
 USER root
 
-# libav-tools for matplotlib anim
+# ffmpeg for matplotlib anim
 RUN apt-get update && \
-    apt-get install -y --no-install-recommends libav-tools && \
+    apt-get install -y --no-install-recommends ffmpeg && \
     apt-get clean && \
     rm -rf /var/lib/apt/lists/*