Fix bug where ODP page numbers would be hidden.

In a9c4f35245d058982a4366d826c42afb41d12ba6,
the regex for checking whether the attribute
presentation:class matches an approved set
of values had an extra ' at the end, which
meant that frames with class 'page-number'
were being dropped.
This is now fixed.
Merge pull request #254 from adityab/pagenumber-fix

Fix bug where ODP page numbers would be hidden.