LanguageCategoryDisplay: Use :last-child instead of :last-of-type

Both of them are not supported natively on IE<=8, but jQuery only
includes a polyfill for the latter since version 1.9, and MediaWiki is
still using 1.8.3. :last-child has a polyfill since jQuery 1.1.4.

http://api.jquery.com/last-child-selector/
http://api.jquery.com/last-of-type-selector/