My approach to Rails app versioning. Keep up with your VCS tags and forget about the rest! (git shown here)

application.rb

# Only attempt update on local machine
if Rails.env.development?
  # Update version file from latest git tag
  File.open('config/version', 'w') do |file|
    file.write `git describe --tags --always` # or equivalent
  end
end

config.version = File.read('config/version')