Minor error fix for upgrades from 5.1.3 or earlier

Closes #256 and #199.

Full credit for locating the fix goes to @LZZZZ.

install.php

@@ -593,7 +593,7 @@ if (file_exists($config['has_installed'])) {
 			  	`extra` varchar(200),
 			  	`text` varchar(255),
 			  	`created_at` int(11),
-			  	PRIMARY KEY (`cookie`,`extra`),
+			  	PRIMARY KEY (`cookie`,`extra`)
 				) ENGINE=MyISAM DEFAULT CHARSET=utf8mb4;') or error(db_error());
 		case false:
 			// TODO: enhance Tinyboard -> vichan upgrade path.