Removing encoding parameter from open from setup.py

This change fixes the following error while installing this package:

    Traceback (most recent call last):
      File "<string>", line 17, in <module>
      File "/tmp/pip-i50wBG-build/setup.py", line 5, in <module>
        readme = open('README.rst', encoding='utf-8')
    TypeError: 'encoding' is an invalid keyword argument for this function