setup.py needs to specify encoding when reading README.rst

I've found another issue with the recent changes to setup.py (https://github.com/crsmithdev/arrow/commit/2e5031b473bcf10b99b9d7ff8a7e72499c412c07)



Depending on the current locale, installation fails when setup.py attempts to read README.rst

```

Collecting arrow<1,>=0 (from this-is-redacted==1.2.3->also-redacted==4.5.6)

  Downloading https://files.pythonhosted.org/packages/90/7f/7fd8accc23aa507d215b5f07b509661c31583010d8276ba844896ee1f44b/arrow-0.14.1.tar.gz (102kB)

    Complete output from command python setup.py egg_info:

    Traceback (most recent call last):

      File "<string>", line 1, in <module>

      File "/tmp/pip-build-3mcnwe97/arrow/setup.py", line 7, in <module>

        readme = f.read()

      File "/apps/api/venv_py3/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 0xec in position 3701: ordinal not in range(128)

```



Also see:

https://github.com/crsmithdev/arrow/issues/202

https://github.com/crsmithdev/arrow/issues/208

https://github.com/crsmithdev/arrow/issues/224

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