Prefer `be_falsey` over `be_falsy`

* In RSpec 3, `be_false` and `be_true` are deprecated in favor of
  `be_falsey` and `be_truthy`
* Source: https://github.com/rspec/rspec-expectations/issues/283
* `be_falsy` works but is just an alias for `be_falsey` so we should prefer the
  latter