Merge branch 'ruby_2_x' into 'master'

Downgrade ruby version to 2.4.4 because of a PollMailbox job bug

See merge request !9

Dockerfile

-FROM ruby:2.5.0
+FROM ruby:2.4.4
 MAINTAINER web-services-core <web-services-core@cern.ch>
 
 ### Install prerequisites #############################
@@ -14,7 +14,7 @@ RUN apt update && \
     # Install image utilities
     advancecomp jpegoptim gifsicle libjpeg-progs libpng-dev optipng pngcrush pngquant jhead gsfonts \
     # Install nodejs and npm
-    nodejs npm && \
+    nodejs && \
     # Install svgo
     npm install -g svgo && \
     # Install pngout