Update RCTActionSheetManager.m

Summary:
We use [self bridge] in this class but weren't synthesizing the bridge getters/setters.

Fixes https://github.com/facebook/react-native/issues/7890
Closes https://github.com/facebook/react-native/pull/7909

Differential Revision: D3390538

fbshipit-source-id: d75a1976af01b21887ddad5060ce635cf84abadd

Libraries/ActionSheetIOS/RCTActionSheetManager.m

@@ -27,6 +27,8 @@
 
 RCT_EXPORT_MODULE()
 
+@synthesize bridge = _bridge;
+
 - (dispatch_queue_t)methodQueue
 {
   return dispatch_get_main_queue();