Update admin.php to use latest file.

The file I previously updated didn't match the file of the plugin when downloaded.

The previous file doesn't work with the old pull request, this is the full update which works fine in 3.5.
Merge branch 'patch-2' of https://github.com/elliott-stocks/wp-gocardless

Conflicts:
	wp-content/plugins/gocardless/admin.php