Adds flutter_maps_firestore to travis config (#96)

flutter_maps_firestore/test/widget_test.dart

@@ -0,0 +1,9 @@
+// Copyright 2019 The Flutter team. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  testWidgets('This test always passes', (WidgetTester tester) async {});
+}