Fix supported Swift version to 4.0.0

Author: mendesbarreto

RxKeyboard.podspec

@@ -11,8 +11,8 @@ Pod::Spec.new do |s|
   s.frameworks       = 'UIKit', 'Foundation'
   s.requires_arc     = true
 
-  s.dependency 'RxSwift', '>= 4.1.2'
-  s.dependency 'RxCocoa', '>= 4.1.2'
+  s.dependency 'RxSwift', '>= 4.0.0'
+  s.dependency 'RxCocoa', '>= 4.0.0'
 
   s.ios.deployment_target = '8.0'
 end