version_reporter.rb

# An example Jekyll generator. Utilizes the new plugin system.
#
# 1. Make a _plugins directory in your jekyll site, and put this class in a file there.
# 2. Upon site generation, version.html will be created in your root destination with
# #  the version of Jekyll that generated it

module Jekyll
  class VersionReporter < Generator
    safe true

    def generate(site)
      filename = 'version.html'
      File.open(File.join(site.config["destination"], filename), 'w') do |f|
        f.write(generate_report(site))
      end
      site.pages << Page.new(site, site.dest, '/', filename)
    end

    private

    def generate_report(site)
      "Site generated with Jekyll version: #{Jekyll::VERSION}"
    end

  end
end

I forked and fixed the problem (wish I could submit a PR across gists) but the issue is that you have to add the page into Jekyll's site.pages array after you create the file.

site.pages << Page.new(site, site.dest, '/', filename)

https://gist.github.com/jasonrhodes/7226155