Rake task to update a Gemfile using currently installed gem versions. This way if no version is specified, it will sub in the one installed on system.

gemfile.rake

require 'bundler'
require 'bundler/dsl'
require 'fileutils'
require 'tempfile'

namespace :gemfile do
  
  task :fresh do    
    builder  = Freshie.new
    tempfile = Tempfile.new("#{builder.gemfile}.temp")
    File.open( builder.gemfile, "r") do |file|
      file.each_line do |line|
        if dep = builder.find_match( line )
          pad  = line.rindex(/gem/)
          repl = "gem #{dep.first.inspect}, \"= #{dep.last}\""
          line.gsub!(/^\s?[\s+]?(gem.*)$/, repl.rjust( (repl.size + pad), " " ))
        end
        tempfile.puts line
      end
    end
    FileUtils.mv(tempfile.path, builder.gemfile)
  end
  
end

class Freshie
  
  attr_accessor :dependencies, :gemfile, :system_gems
  
  def initialize( gemfile = nil )
    @gemfile ||= File.expand_path("../../../", __FILE__) << "/Gemfile"
    @dependencies = []
    builder       = Bundler::Dsl.new
    builder.instance_eval( Bundler.read_file(@gemfile.to_s), @gemfile.to_s, 1 )
    
    deps     = builder.instance_variable_get('@dependencies')    
    deps.each{ |dep| add_dependency(dep) }
    @dependencies.uniq!
  end
  
  def add_dependency( dep )
    return true unless dep.source.nil? && dep.requirement.to_s.match(/\=\ 0$/)
    sys_version = system_gems[dep.name]
    return true if sys_version.nil?
    @dependencies << [ dep.name, sys_version ]
  end
  
  def system_gems
    @system_gems ||= (`gem list`).split("\n").inject({}) do |hash, spec|
      parts    = spec.strip.match(/^([\w+|\-?]+\s{1})(.*)$/ix).captures
      versions = parts.map!(&:strip).last.split(",").map!(&:strip)
      hash.merge!( parts.first => versions.first.gsub(/[^.\d\w]/,'') )
    end
  end
  
  def find_match( line )
    matches = line.match(/(gem)\s?(\S+)/i)
    return false if matches.nil?
    spec = matches.captures.last.gsub!(/[^\w|\-]/, '')
    dep  = @dependencies.detect{ |d| spec == d.first }
    
    ( dep.nil? ? false : dep )
  end
  
end