Unify ERB version definitions

Gemfile

@@ -1,4 +1,6 @@
 source 'https://rubygems.org'
 
+gemspec
+
 gem 'rake'
 gem 'test-unit'

erb.gemspec

@@ -1,6 +1,13 @@
+begin
+  require_relative 'lib/erb/version'
+rescue LoadError
+  # for Ruby core repository
+  require_relative 'version'
+end
+
 Gem::Specification.new do |spec|
   spec.name          = 'erb'
-  spec.version       = '2.2.0'
+  spec.version       = ERB.const_get(:VERSION, false)
   spec.authors       = ['Masatoshi SEKI']
   spec.email         = ['seki@ruby-lang.org']
 

lib/erb.rb

@@ -12,7 +12,8 @@
 #
 # You can redistribute it and/or modify it under the same terms as Ruby.
 
-require "cgi/util"
+require 'cgi/util'
+require 'erb/version'
 
 #
 # = ERB -- Ruby Templating
@@ -260,7 +261,7 @@ class ERB
 
   # Returns revision information for the erb.rb module.
   def self.version
-    "erb.rb [2.2.0 #{ERB::Revision.split[1]}]"
+    "erb.rb [#{VERSION} #{ERB::Revision.split[1]}]"
   end
 end
 

lib/erb/version.rb

@@ -0,0 +1,5 @@
+# frozen_string_literal: false
+class ERB
+  VERSION = '2.2.0'
+  private_constant :VERSION
+end