Fix deprecated plugin fields usage

pubspec.yaml

@@ -7,6 +7,7 @@ repository: https://github.com/juliuscanute/qr_code_scanner
 
 environment:
   sdk: ">=2.3.0 <3.0.0"
+  flutter: ^1.10.0
 
 dependencies:
   flutter:
@@ -17,5 +18,9 @@ dev_dependencies:
 
 flutter:
   plugin:
-    androidPackage: net.touchcapture.qr.flutterqr
-    pluginClass: FlutterQrPlugin
+    platforms:
+      android:
+        package: net.touchcapture.qr.flutterqr
+        pluginClass: FlutterQrPlugin
+      ios:
+        pluginClass: FlutterQrPlugin