Bundle Latest REXML

Sam Ruby has been doing a bang-up job fixing the bugs in REXML.
Who knows when these improvements will trickle down to vendor distributions of Ruby.
In the meantime, let's bundle the latest version of REXML with Instiki.
We check the version number of the bundled REXML against that of the System REXML, and use whichever is later.

vendor/plugins/maruku/tests/unittest/attributes/attributes.md

@@ -79,4 +79,27 @@ Header with attributesHeader with attributesHeader no attributesParagraph with a
 <p>{:hello: .chello}</p>
 
 *** Output of Markdown.pl (parsed) ***
-Error: #<NoMethodError: private method `write_children' called for <div> ... </>:REXML::Element>
+<div>
+ <h2>
+  Header with attributes {#header1} 
+ </h2>
+ <h3>
+  Header with attributes ### {#header2}
+ </h3>
+ <h3>
+  Header no attributes
+ </h3>
+ <p>
+  {:warn2}Paragraph with a. {#par1}
+ </p>
+ <p>
+  Paragraph with 
+  <em>
+   emphasis
+  </em>
+  {:hello notfound} {#par2}
+ </p>
+ <p>
+  {:hello: .chello}
+ </p>
+</div>