Refactor factories

* Update `factory_girl_rails` gem so we can use `FactoryGirl.lint`;
* Add `FactoryGirl.lint` to run before the test suite to check for invalid factories before running the test suite;
* Add `database_cleaner` gem to run before each test. This way we will have isolated tests.
* Fix AdminUser and Answer factories (FactoryGirl.lint was breaking on them).