Simplify installation and add tests to examples

We should replace requirements.txt with a Pipfile to make the installation of the [examples](https://github.com/graphql-python/graphene-sqlalchemy/tree/master/examples) even easier with Pipenv (the READMEs the should be adapted, too).

And we should add tests for the examples (e.g. running this [`default_query`](https://github.com/graphql-python/graphene-sqlalchemy/blob/1033de566244ce340f900dfd3889ad0ff958e804/examples/flask_sqlalchemy/app.py#L12)) that can be run with Travis so we notice when this breaks again, and that also demonstrate how the example servers can be consumed by a client.

首先檢查 requirements.txt、範例 README，以及 examples/flask_sqlalchemy/app.py 中 default_query 附近的程式碼。查看 Travis 目前如何執行專案，接著定義從用戶端測試伺服器的範例測試。當安裝使用 Pipfile、README 已更新，且範例測試在 Travis 中執行時，即表示完成。

由索引模型根據 Issue 內容生成。