Adopt flutter version 0.3.1

[gen0083]

Overview

This PR resolve some errors on Flutter version 0.3.1.

Environment

Flutter 0.3.1 • channel beta • https://github.com/flutter/flutter.git Framework • revision 12bbaba9ae (11 days ago) • 2018-04-19 23:36:15 -0700 Engine • revision 09d05a3891 Tools • Dart 2.0.0-dev.48.0.flutter-fe606f890b

Detail

• commented out about release-key signing information in build.gradle see issue #64
• update cloud_firestore version caused SDK version constraint
• add mockito dependency for test
• update google_sign_in version to fix type mismatch on run-time
• update shared_preferences version to fix type mismatch on run-time
• fix type mismatch error telled from dart analysis