fixed bug in metro._get_top_specific_artists - replaced blank 'playcount...

Though in the Last.fm API documentation there is a playcount field listed in the "Sample Responses" in geo.getMetroHypeArtists, geo.getMetroUniqueArtists, and geo.getMetroArtistChart, not only is this field blank in all of the sample responses, it doesn't even exist in the actual API output. As an example, compare the [sample response of geo.getMetroHypeArtistChart](http://www.last.fm/api/show/geo.getMetroHypeArtistChart) to the [actual XML output](http://i.imgur.com/o75YSoD.png).

Though the other two methods have neither a listeners or a playcount field, the actual API output for geo.getMetroArtistChart has a non-empty "listeners" field which it uses for a ranking metric instead of playcount. I updated Metro._ get_top_specific_artists() accordingly.