Backport Eloquent BelongsToMany::sync function signature

Backport new \Illuminate\Database\Eloquent\Relations\BelongsToMany::sync function signature from commit https://github.com/laravel/framework/commit/ac5c270151c7d646e698391e1bd921e054e66299 (included in Laravel v4.1.26)

This allows to fix the following errors when using a BelongsToMany relation:

"[ErrorException]                                                                                                                     
Declaration of Jenssegers\Mongodb\Relations\BelongsToMany::sync() should be compatible with Illuminate\Database\Eloquent\Relations\BelongsToMany::sync($ids, $detaching = true)"