make a Gemfile from currently installed gems

make_gemfile_from_gems.rb

#!/usr/bin/env ruby

gems = `gem list`
puts "source https://rubygems.org"

gems.each_line{|line|
  (gem, versions) = line.split(' ')
  highest_version = versions.split(',').first.gsub(/[\(\)]/, '')
  puts "gem '#{gem}', '#{highest_version}'"
}