Minor: change bukkit service to return a plain collection of Player

- List is read-only and can be easily "upcast" to Collection<Player>
- Remove awkward hack that was needed for mocking the method previously (issues with generics)

src/main/java/fr/xephi/authme/service/BukkitService.java

@@ -233,8 +233,9 @@ public class BukkitService implements SettingsDependent {
      *
      * @return collection of online players
      */
-    public Collection<? extends Player> getOnlinePlayers() {
-        return Bukkit.getOnlinePlayers();
+    @SuppressWarnings("unchecked")
+    public Collection<Player> getOnlinePlayers() {
+        return (Collection<Player>) Bukkit.getOnlinePlayers();
     }
 
     /**