remove_role / revoke produces invalid SQL error if role_name argument is a symbol

If you do a ```user_object.add_role :admin``` it succeeds but if you do ```user_object.remove_role :admin``` it'll produce something below:

```
ActiveRecord::StatementInvalid: SQLite3::SQLException: no such column: roles.admin: SELECT "roles".* FROM "roles" INNER JOIN "users_roles" ON "roles"."id" = "users_roles"."role_id" WHERE "users_roles"."user_id" = 10 AND "roles"."name" = "roles"."admin"
```

Attached code is the simple fix that performs ```role_name.to_s``` to ensure a symbol can be accepted. **Not sure on the resource argument. It may require tweaking as well. See ```def add_role``` for reference.**