[ruby/erb] Publish constant ERB::VERSION

Even cgi.gem publicly defines CGI::VERSION today. It's just weird that
ERB::VERSION is kept private at this point.

https://github.com/ruby/erb/commit/46801cbd47

lib/erb.gemspec

@@ -7,7 +7,7 @@ end
 
 Gem::Specification.new do |spec|
   spec.name          = 'erb'
-  spec.version       = ERB.const_get(:VERSION, false)
+  spec.version       = ERB::VERSION
   spec.authors       = ['Masatoshi SEKI', 'Takashi Kokubun']
   spec.email         = ['seki@ruby-lang.org', 'k0kubun@ruby-lang.org']
 

lib/erb/version.rb

@@ -1,5 +1,4 @@
 # frozen_string_literal: true
 class ERB
   VERSION = '4.0.4'
-  private_constant :VERSION
 end