Add Brakeman by default to new apps

Brakeman is a great way to prevent common security vulnerabilities in Rails from going into production. We should be encouraging that for new applications. Especially for newcomers to the framework who might otherwise make rookie mistakes. So let's add it in the same way we just added Rubocop. --skip-brakeman if you don't want it, otherwise we add to default gemfile and create a default binstub in bin/brakeman.