Update Plupload Silverlight binary to 1.5.8 in the 3.8 branch.

Built from https://develop.svn.wordpress.org/branches/3.8@27920


git-svn-id: http://core.svn.wordpress.org/branches/3.8@27750 1a063a9b-81f0-0310-95a4-ce76da25c4cd