Serve error: undefined method `post_read' for class `Jekyll::Document'

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Thanks, ashmaroli, for the reply! When I run jekyll serve --trace, I get the following output:

Configuration file: C:/cygwin64/home/gw/portfolio/_config.yml
C:/RailsInstaller/Ruby2.3.0/lib/ruby/gems/2.3.0/gems/jekyll-multiple-languages-2.0.3/lib/jekyll-multiple-languages/document.rb:15:in `<class:Document>': undefined method `post_read' for class `Jekyll::Document' (NameError)
        from C:/RailsInstaller/Ruby2.3.0/lib/ruby/gems/2.3.0/gems/jekyll-multiple-languages-2.0.3/lib/jekyll-multiple-languages/document.rb:3:in `<module:Jekyll>'
        from C:/RailsInstaller/Ruby2.3.0/lib/ruby/gems/2.3.0/gems/jekyll-multiple-languages-2.0.3/lib/jekyll-multiple-languages/document.rb:2:in `<top (required)>'
        from C:/RailsInstaller/Ruby2.3.0/lib/ruby/2.3.0/rubygems/core_ext/kernel_require.rb:55:in `require'
        from C:/RailsInstaller/Ruby2.3.0/lib/ruby/2.3.0/rubygems/core_ext/kernel_require.rb:55:in `require'
        from C:/RailsInstaller/Ruby2.3.0/lib/ruby/gems/2.3.0/gems/jekyll-multiple-languages-2.0.3/lib/jekyll-multiple-languages.rb:3:in `<top (required)>'
        from C:/RailsInstaller/Ruby2.3.0/lib/ruby/2.3.0/rubygems/core_ext/kernel_require.rb:127:in `require'
        from C:/RailsInstaller/Ruby2.3.0/lib/ruby/2.3.0/rubygems/core_ext/kernel_require.rb:127:in `rescue in require'
        from C:/RailsInstaller/Ruby2.3.0/lib/ruby/2.3.0/rubygems/core_ext/kernel_require.rb:40:in `require'
        from C:/RailsInstaller/Ruby2.3.0/lib/ruby/gems/2.3.0/gems/jekyll-3.4.3/lib/jekyll/external.rb:43:in `block in require_with_graceful_fail'
        from C:/RailsInstaller/Ruby2.3.0/lib/ruby/gems/2.3.0/gems/jekyll-3.4.3/lib/jekyll/external.rb:40:in `each'
        from C:/RailsInstaller/Ruby2.3.0/lib/ruby/gems/2.3.0/gems/jekyll-3.4.3/lib/jekyll/external.rb:40:in `require_with_graceful_fail'
        from C:/RailsInstaller/Ruby2.3.0/lib/ruby/gems/2.3.0/gems/jekyll-3.4.3/lib/jekyll/plugin_manager.rb:27:in `require_gems'
        from C:/RailsInstaller/Ruby2.3.0/lib/ruby/gems/2.3.0/gems/jekyll-3.4.3/lib/jekyll/plugin_manager.rb:19:in `conscientious_require'
        from C:/RailsInstaller/Ruby2.3.0/lib/ruby/gems/2.3.0/gems/jekyll-3.4.3/lib/jekyll/site.rb:111:in `setup'
        from C:/RailsInstaller/Ruby2.3.0/lib/ruby/gems/2.3.0/gems/jekyll-3.4.3/lib/jekyll/site.rb:33:in `initialize'
        from C:/RailsInstaller/Ruby2.3.0/lib/ruby/gems/2.3.0/gems/jekyll-3.4.3/lib/jekyll/commands/build.rb:28:in `new'
        from C:/RailsInstaller/Ruby2.3.0/lib/ruby/gems/2.3.0/gems/jekyll-3.4.3/lib/jekyll/commands/build.rb:28:in `process'
        from C:/RailsInstaller/Ruby2.3.0/lib/ruby/gems/2.3.0/gems/jekyll-3.4.3/lib/jekyll/commands/serve.rb:37:in `block (2 levels) in init_with_program'
        from C:/RailsInstaller/Ruby2.3.0/lib/ruby/gems/2.3.0/gems/mercenary-0.3.6/lib/mercenary/command.rb:220:in `block in execute'
        from C:/RailsInstaller/Ruby2.3.0/lib/ruby/gems/2.3.0/gems/mercenary-0.3.6/lib/mercenary/command.rb:220:in `each'
        from C:/RailsInstaller/Ruby2.3.0/lib/ruby/gems/2.3.0/gems/mercenary-0.3.6/lib/mercenary/command.rb:220:in `execute'
        from C:/RailsInstaller/Ruby2.3.0/lib/ruby/gems/2.3.0/gems/mercenary-0.3.6/lib/mercenary/program.rb:42:in `go'
        from C:/RailsInstaller/Ruby2.3.0/lib/ruby/gems/2.3.0/gems/mercenary-0.3.6/lib/mercenary.rb:19:in `program'
        from C:/RailsInstaller/Ruby2.3.0/lib/ruby/gems/2.3.0/gems/jekyll-3.4.3/exe/jekyll:13:in `<top (required)>'
        from C:/RailsInstaller/Ruby2.3.0/bin/jekyll:22:in `load'
        from C:/RailsInstaller/Ruby2.3.0/bin/jekyll:22:in `<main>'```

So it seems to be a problem with the `jekyll-multiple-languages` gem. My problem now is that I don't know how to proceed to fix it, since I'm really rather a newbie at all this. :) I've (at least for now) changed my repo to public in case it helps: https://bitbucket.org/maxstarkenburg/portfolio/overview