Widget visibility: Add support for old-style single widgets

Props @westonruter

Fixes the following notice:
`Undefined offset: 1 at /jetpack/modules/widget-visibility/widget-conditions.php line 251`

This is the line that is erroring
`list( $basename, $suffix ) = explode( "-", $widget_id, 2 );`
The problem is that it assumes the a -number on the end of the widget name

This is not safe as you can widget names like:
`icl_lang_sel_widget`
`recent-comments-2`

For more details, view the original ticket:
http://plugins.trac.wordpress.org/ticket/1979