Let ERB.version just return a version

and deprecate ERB::Revision.

`'$Date::                           $'` has not been working since Git
migration from SVN. I'm sorry.

Because it has been already broken, I'd like to take this opportunity to
make `ERB.version` compatible with `Gem::Version.new`.

Author: k0kubun

lib/erb.rb

@@ -258,10 +258,11 @@
 #
 class ERB
   Revision = '$Date::                           $' # :nodoc: #'
+  deprecate_constant :Revision
 
   # Returns revision information for the erb.rb module.
   def self.version
-    "erb.rb [#{VERSION} #{ERB::Revision.split[1]}]"
+    VERSION
   end
 end