add rake version helpers

Rakefile

@@ -38,11 +38,84 @@ namespace :test do
   end
 end
 
+# Print the current version of this gem or update it.
+#
+# @param [Type] target the new version you want to set, or nil if you only want to show
+def train_version(target = nil)
+  path = 'lib/train/version.rb'
+  require_relative path.sub(/.rb$/, '')
+
+  nu_version = target.nil? ? '' : " -> #{target}"
+  puts "Train: #{Train::VERSION}#{nu_version}"
+
+  unless target.nil?
+    raw = File.read(path)
+    nu = raw.sub(/VERSION.*/, "VERSION = '#{target}'")
+    File.write(path, nu)
+    load(path)
+  end
+end
+
 # Automatically generate a changelog for this project. Only loaded if
 # the necessary gem is installed.
 begin
   require 'github_changelog_generator/task'
-  GitHubChangelogGenerator::RakeTask.new :changelog
+  GitHubChangelogGenerator::RakeTask.new :changelog do |config|
+    require_relative 'lib/train/version'
+    config.future_release = Train::VERSION
+  end
 rescue LoadError
-  puts '>>>>> GitHub Changelog Generator not loaded, omitting tasks'
+end
+
+# Check if a command is available
+#
+# @param [Type] x the command you are interested in
+# @param [Type] msg the message to display if the command is missing
+def require_command(x, msg = nil)
+  return if system("command -v #{x} || exit 1")
+  msg ||= 'Please install it first!'
+  puts "\033[31;1mCan't find command #{x.inspect}. #{msg}\033[0m"
+  exit 1
+end
+
+# Check if a required environment variable has been set
+#
+# @param [String] x the variable you are interested in
+# @param [String] msg the message you want to display if the variable is missing
+def require_env(x, msg = nil)
+  exists = `env | grep "^#{x}="`
+  return unless exists.empty?
+  puts "\033[31;1mCan't find environment variable #{x.inspect}. #{msg}\033[0m"
+  exit 1
+end
+
+# Check the requirements for running an update of this repository.
+def check_update_requirements
+  require_command 'git'
+  require_command 'github_changelog_generator', "\n"\
+    "For more information on how to install it see:\n"\
+    "  https://github.com/skywinder/github-changelog-generator\n"
+  require_env 'CHANGELOG_GITHUB_TOKEN', "\n"\
+    "Please configure this token to make sure you can run all commands\n"\
+    "against GitHub.\n\n"\
+    "See github_changelog_generator homepage for more information:\n"\
+    "  https://github.com/skywinder/github-changelog-generator\n"
+end
+
+# Show the current version of this gem.
+desc 'Show the version of this gem'
+task :version do
+  train_version
+end
+
+# Update the version of this gem and create an updated
+# changelog. It covers everything short of actually releasing
+# the gem.
+desc 'Bump the version of this gem'
+task :bump_version, [:version] do |_, args|
+  v = args[:version] || ENV['to']
+  fail "You must specify a target version!  rake release[1.2.3]" if v.empty?
+  check_update_requirements
+  train_version(v)
+  Rake::Task['changelog'].invoke
 end