Fix broken style sheet URL

GitHub is now including the full URL to the style sheet in their JSON response, so the resolved URL was https://gist.github.comhttp://gist.github.com... which broke the appearance of gists.

Props to @wpbacon. Fixes bradyvercher#42

Author: GaryJones

includes/class-gistpress.php

@@ -378,7 +378,7 @@ public function get_gist_html( $url, array $args ) {
 
 				// Update the style sheet reference.
 				if ( ! empty( $json->stylesheet ) ) {
-					update_option( 'gistpress_stylesheet', 'https://gist.github.com' . $json->stylesheet );
+					update_option( 'gistpress_stylesheet', $json->stylesheet );
 				}
 			}