fixed flow error introduced with Sldier

Summary:cc mkonicek
Closes https://github.com/facebook/react-native/pull/6838

Differential Revision: D3144492

fb-gh-sync-id: 49eeb95fd2ccaa1b77905bd258b36f48c84463e4
fbshipit-source-id: 49eeb95fd2ccaa1b77905bd258b36f48c84463e4

Libraries/react-native/react-native.js.flow

@@ -44,6 +44,7 @@ var ReactNative = Object.assign(Object.create(require('react')), {
   ScrollView: require('ScrollView'),
   SegmentedControlIOS: require('SegmentedControlIOS'),
   SliderIOS: require('SliderIOS'),
+  Slider: require('Slider'),
   SnapshotViewIOS: require('SnapshotViewIOS'),
   StatusBar: require('StatusBar'),
   Switch: require('Switch'),