Avoid UnicodeDecodeError in setup.py

[muupan]

pip install -e . raises UnicodeDecodeError when LC_CTYPE is POSIX.

      Traceback (most recent call last):
        File "<string>", line 1, in <module>
        File "/home/user/setup.py", line 31, in <module>
          long_description=open('README.md').read(),
        File "/usr/lib/python3.5/encodings/ascii.py", line 26, in decode
          return codecs.ascii_decode(input, self.errors)[0]
      UnicodeDecodeError: 'ascii' codec can't decode byte 0xe2 in position 2004: ordinal not in range(128)

To reproduce this error, LC_ALL=POSIX pip install -e ..

This PR fixes this error by specifying the encoding when reading README.md.

[toslunar]

LGTM