Update to etherpad 1.9.6

The changelog [0] indicates this is largely a bugfix and image build
update. We update our image build accordingly.

[0] https://github.com/ether/etherpad-lite/blob/v1.9.6/CHANGELOG.md

Change-Id: I439aa41eaee1dd7825d41ea3da9b1903fa27fa44

docker/etherpad/Dockerfile

@@ -85,6 +85,7 @@ RUN mkdir -p "${EP_DIR}" && chown etherpad:etherpad "${EP_DIR}"
 # https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=863199
 RUN export DEBIAN_FRONTEND=noninteractive; \
     mkdir -p /usr/share/man/man1 && \
+    npm install npm@6 -g  && \
     apt-get -qq update && \
     apt-get -qq dist-upgrade && \
     apt-get -qq --no-install-recommends install \
@@ -101,7 +102,7 @@ USER etherpad
 
 RUN git clone https://github.com/ether/etherpad-lite ${EP_DIR}
 WORKDIR "${EP_DIR}"
-RUN git checkout v1.9.5
+RUN git checkout v1.9.6
 
 # Plugins must be installed before installing Etherpad's dependencies, otherwise
 # npm will try to hoist common dependencies by removing them from