Update datatables-fixedheader to 2.1.0

Update DataTables extra called FixedHeader. FixedHeader is an official
/ plugin found at http://datatables.net/extras/fixedheader/. Source
downloaded from http://datatables.net/download/. npm test passed.
Merge pull request #2644 from moderation/master

Update datatables-fixedheader to 2.1.0