Set Twenty Fifteen as the new default theme. see #29799.

git-svn-id: https://develop.svn.wordpress.org/trunk@29895 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/class-wp-theme.php

@@ -44,6 +44,7 @@ final class WP_Theme implements ArrayAccess {
 		'twentytwelve'   => 'Twenty Twelve',
 		'twentythirteen' => 'Twenty Thirteen',
 		'twentyfourteen' => 'Twenty Fourteen',
+		'twentyfifteen'  => 'Twenty Fifteen',
 	);
 
 	/**