Adding Shrine to travis script. (#6)

shrine/test/main_test.dart

@@ -1 +1,11 @@
-void main() {}
+// Copyright 2018 The Chromium Authors. 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() {
+  group('Unit tests', () {
+    test('This test always passes', () {});
+  });
+}