Assets do not compile for 9.3.0.pre for CentOS6

Master builds are failing with:

$ NODE_ENV=production PATH=/opt/gitlab/embedded/bin:/builds/gitlab/omnibus-gitlab/gems/ruby/2.3.0/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin RAILS_ENV=production SKIP_STORAGE_VALIDATION=true USE_DB=false /opt/gitlab/embedded/bin/bundle exec rake gitlab:assets:compile

Output:

(nothing)

Error:

warning fsevents@1.0.17: The platform "linux" is incompatible with this module.
warning Optional dependency "compression-webpack-plugin#node-zopfli" not installed
WARNING: This version of GitLab depends on gitlab-shell 5.0.5, but you're running Unknown. Please update gitlab-shell.
/var/cache/omnibus/src/gitlab-rails/node_modules/.bin/webpack --config /var/cache/omnibus/src/gitlab-rails/config/webpack.config.js --bail
/var/cache/omnibus/src/gitlab-rails/node_modules/compression-webpack-plugin/index.js:20
throw new Error("node-zopfli not found");
^

Error: node-zopfli not found
at new CompressionPlugin (/var/cache/omnibus/src/gitlab-rails/node_modules/compression-webpack-plugin/index.js:20:11)
at Object.<anonymous> (/var/cache/omnibus/src/gitlab-rails/config/webpack.config.js:223:7)
at Module._compile (module.js:570:32)
at Object.Module._extensions..js (module.js:579:10)
at Module.load (module.js:487:32)
at tryModuleLoad (module.js:446:12)
at Function.Module._load (module.js:438:3)
at Module.require (module.js:497:17)
at require (internal/module.js:20:19)
at requireConfig (/var/cache/omnibus/src/gitlab-rails/node_modules/webpack/bin/convert-argv.js:96:18)
rake aborted!
Command failed with status (1): [/var/cache/omnibus/src/gitlab-rails/node_m...]
/opt/gitlab/embedded/bin/bundle:22:in `load'
/opt/gitlab/embedded/bin/bundle:22:in `<main>'
Tasks: TOP => gitlab:assets:compile => webpack:compile
(See full trace by running task with --trace)

@mikegreiling @timzallmann can you take a look at why CentOS 6 does not have node-zopfli?

Introduced with https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/11798

If we don't find a quick solution, we should revert the MR.