Add support for old strscan

If we support old strscan, users can also use strscan installed as a
default gem.

lib/rexml/parsers/baseparser.rb

@@ -7,6 +7,17 @@
 
 module REXML
   module Parsers
+    if StringScanner::Version < "3.0.8"
+      module StringScannerCaptures
+        refine StringScanner do
+          def captures
+            values_at(*(1...size))
+          end
+        end
+      end
+      using StringScannerCaptures
+    end
+
     # = Using the Pull Parser
     # <em>This API is experimental, and subject to change.</em>
     #  parser = PullParser.new( "<a>text<b att='val'/>txet</a>" )

rexml.gemspec

@@ -55,5 +55,5 @@ Gem::Specification.new do |spec|
 
   spec.required_ruby_version = '>= 2.5.0'
 
-  spec.add_runtime_dependency("strscan", ">= 3.0.9")
+  spec.add_runtime_dependency("strscan")
 end