Switch Twitter oEmbed to SSL due to a Twitter API change that broke embedding across all WordPress sites.

Merges [26967] to the 3.8 branch.

props yurivictor, cojennin.
fixes #26844.

Built from https://develop.svn.wordpress.org/branches/3.8@26969


git-svn-id: http://core.svn.wordpress.org/branches/3.8@26848 1a063a9b-81f0-0310-95a4-ce76da25c4cd