Update setup.py

[d-huck]

When attempting to install the mixcloud library using pip 10.0+, the following error occurs:

➜ pip install mixcloud
Collecting mixcloud
  Using cached https://files.pythonhosted.org/packages/e3/9e/70aedb77e9b5d99b54dd569d0ba2357cb64909b20805f459f90d45c426c6/mixcloud-0.4.0.tar.gz
    Complete output from command python setup.py egg_info:
    Traceback (most recent call last):
      File "<string>", line 1, in <module>
      File "/tmp/pip-install-v5rwvzwv/mixcloud/setup.py", line 1, in <module>
        from pip.req import parse_requirements
    ModuleNotFoundError: No module named 'pip.req'

    ----------------------------------------
Command "python setup.py egg_info" failed with error code 1 in /tmp/pip-install-v5rwvzwv/mixcloud/

Since pip 10.0, pip.reqs is no longer included and requirements are now preferred to be inside of the install_requires kwarg inside of the setuptools.setup(). I have moved the contents of requirements_dev.txt to this kwarg and removed the line importing pip.reqs.