Splat the arguments to strong_parameters#permit, fixes strong_parameters gem support

Fixes devise_invitable to work exactly the same way as devise now does, which fixes strong_parameters gem support, and also works exactly the same in rails4.

See here for the related devise issue:
plataformatec/devise#2716

See here for the related devise commits:
plataformatec/devise@e445039716c3397e0ef286257bb9bfffe93f2d67