fixbug

fix bug 

Traceback (most recent call last):
      File "<string>", line 1, in <module>
      File "/private/var/folders/23/hvd2rp8s0fx60p0h0klp6qvw0000gn/T/pip-build-CrQhiO/python-weixin/setup.py", line 7, in <module>
        with open('requirements.txt') as f:
    IOError: [Errno 2] No such file or directory: 'requirements.txt'