Update relation columns to match database columns

[amrography]

Fix issue https://github.com/codebyray/laravel-review-rateable/issues/50

{
    "message": "SQLSTATE[42S22]: Column not found: 1054 Unknown column 'reviewable_type' in 'field list' (Connection: mysql, SQL: select distinct `reviewable_type` from `reviews`)",
    "exception": "Illuminate\\Database\\QueryException",
    "file": "/Users/amrokhaled/Code/damg/api/vendor/laravel/framework/src/Illuminate/Database/Connection.php",
    "line": 795
}