Require at least requests 1.0.0

Prior to requests 1.0.0, [Response.json](http://docs.python-requests.org/en/v0.14.2/api/?highlight=json#requests.Response.json) was a property. Response.json [was made callable](https://github.com/kennethreitz/requests/blob/master/HISTORY.rst#100-2012-12-17) in [1.0.0](http://docs.python-requests.org/en/v1.0.0/api/?highlight=json#requests.Response.json).