Updated FixedColumns to version 2.5.0.dev and also updated Bootstrap 2 integration CSS

Even though the file name of FixedColumns was changed in this [commit](https://github.com/DataTables/FixedColumns/commit/ad47ed8e8775343ae25a71fdad326d3950e6f62c) the original file name FixedColumns.js was retained. Moving forward it would be better to follow the DataTables file naming convention.

The Bootstrap 2 integration CSS file was updated to the [latest file](https://github.com/DataTables/Plugins/blob/master/integration/bootstrap/2/dataTables.bootstrap.css) in the DataTables Plugins repository.