attempt to fix installation with cocoapods (#278)

* attempt to fix installation with cocoapods

* Remove unnecessary header imports

ios/RNFIRMessaging.h

 
 #import <UIKit/UIKit.h>
 
-@import FirebaseCore;
+#import <FirebaseCore/FIRApp.h>
 
 #import <React/RCTBridgeModule.h>
 
@@ -13,3 +13,4 @@ extern NSString *const FCMNotificationReceived;
 @property (nonatomic, assign) bool connectedToFCM;
 
 @end
+

ios/RNFIRMesssaging.m

@@ -5,8 +5,8 @@
 #import <React/RCTUtils.h>
 
 @import UserNotifications;
-@import FirebaseMessaging;
-@import FirebaseInstanceID;
+#import <FirebaseMessaging/FirebaseMessaging.h>
+#import <FirebaseInstanceID/FirebaseInstanceID.h>
 
 #if __IPHONE_OS_VERSION_MIN_REQUIRED < __IPHONE_8_0