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.