Fatal Error (E_ERROR): Call to a member function validate() on a non-object

## Description

When trying to create a new user, the user controller is trying to call validate() on the userRegistrationValidator object, which doesn't get set in the user service provider.

Introduced by #77.

## Details

- URL: `/users`
- Console Error: 
Fatal Error (E_ERROR): `Call to a member function validate() on a non-object {"code":1,"message":"Call to a member function validate() on a non-object","file":"/vagrant/vendor/synapsestudios/synapse-base/src/Synapse/User/UserController.php","line":62}`