Fix MorphToMany relation not using Eloquent\Model@getMorphClass()

Eloquent\Relations\MorphTo uses getMorphClass() method on Eloquent\Model to determine a model's morphClass, but Eloquent\Relations\MorphToMany still uses PHP builtin get_class. 

getMorphClass() method on Eloquent\Model originally introduced in b41bf57f65f32572ba3113910213f4d1077a7578

Signed-off-by: Jason Urasky <jurasky@gmail.com>