Updates Plupload to version 1.5.4.

The JavaScript, Silverlight, and SWF files had 
changes. The stylesheets did not have any changes.

Plupload release notes:
  1.5.4: http://j.mp/Plupload154
  1.5.3: http://j.mp/Plupload153
Merge branch 'master' of github.com:matiaskorhonen/plupload-rails