Fix unclosed <div> in labeledslider.

This commit fixes an unclosed <div> in labeledslider (class
"ui-slider-labels"). Without, it make chromium choke on the generated
source in XHTML files with the following error message:

"Uncaught SyntaxError: Failed to set the 'innerHTML' property on
'Element': The provided markup is invalid XML, and therefore cannot be
inserted into an XML document."
Merge pull request #14 from mgudemann/fix-xhtml-labeledslider

Fix unclosed <div> in labeledslider.