title: RxSwift crashes when running UnitTest on XCode 9.3
body: <p>did anyone face RxSwift crash on XCode 9.3? here is the function that causes the problem </p>
  
  <pre><code>func setupValidation(result: Variable&lt;ValidationState&gt;, testValues: Observable&lt;Bool&gt;...) {
  let disposeBag: DisposeBag = DisposeBag()
  Validation
    .combine(testValues)
    .bind(to: result)
    .disposed(by: disposeBag)}
  </code></pre>
  
  <p>here is the error:</p>
  
  <blockquote>
    <p>Undefined symbols for architecture arm64:   "(extension in
    RxCocoa):RxSwift.ObservableType.bind(to: RxSwift.Variable) ->
    RxSwift.Disposable"</p>
  </blockquote>