Fix minify error

etherpad.js does not get minified correctly with minify@5.2.0. Instead 
of the minified js, there is just 'Promise { <pending> }'. minify@5.1.1 
works correctly

pica-etherpad/Dockerfile

@@ -36,7 +36,7 @@ RUN curl -SL https://github.com/ether/etherpad-lite/archive/${ETHERPAD_VERSION_B
 COPY ./landing-page ./src/templates/landing-page
 
 # Build and replace landing page
-RUN npm install minify@5 && \
+RUN npm install minify@5.1.1 && \
     ./src/templates/landing-page/build/build.sh && \
     cp ./src/templates/landing-page/index.html src/templates/index.html && \
     cp -r ./src/templates/landing-page/static/* src/static/