Remove updating of rubygems (#896)

This adds an additional layer of 20mb for no reason. We don't need to be
running the latest version of rubygems all the time.

Even if we need to update rubygems, it should be updated in https://github.com/discourse/docker-ruby

image/base/Dockerfile

@@ -127,9 +127,6 @@ RUN /tmp/install-redis
 ADD install-oxipng /tmp/install-oxipng
 RUN /tmp/install-oxipng
 
-RUN echo 'gem: --no-document' >> /usr/local/etc/gemrc &&\
-    gem update --system
-
 RUN gem install pups --force &&\
     mkdir -p /pups/bin/ &&\
     ln -s /usr/local/bin/pups /pups/bin/pups