Use regular player name when tab completing /ma kick and /ma restore.

Since the actual execution of these commands use the regular player name
instead of the display name, the tab completion is useless if it doesn't
also use the regular player name.

Fixes #589

src/main/java/com/garbagemule/MobArena/commands/admin/KickCommand.java

@@ -52,8 +52,8 @@ public class KickCommand implements Command
         List<Player> players = am.getAllPlayers();
 
         return players.stream()
-            .filter(p -> p.getDisplayName().toLowerCase().startsWith(prefix))
-            .map(Player::getDisplayName)
+            .filter(p -> p.getName().toLowerCase().startsWith(prefix))
+            .map(Player::getName)
             .collect(Collectors.toList());
     }
 }

src/main/java/com/garbagemule/MobArena/commands/admin/RestoreCommand.java

@@ -55,8 +55,8 @@ public class RestoreCommand implements Command
         Collection<? extends Player> players = am.getPlugin().getServer().getOnlinePlayers();
 
         return players.stream()
-            .filter(p -> p.getDisplayName().toLowerCase().startsWith(prefix))
-            .map(Player::getDisplayName)
+            .filter(p -> p.getName().toLowerCase().startsWith(prefix))
+            .map(Player::getName)
             .collect(Collectors.toList());
     }
 }