Replace non-ASCII apostrophe

[Alphadash]

In some environments, a non-ASCII character causes a failure when pip installing datadog.
I could not replicate on my local OSX machine, but I have a Docker image based on Ubuntu 14.04.5 LTS that fails reliably with the character in the README.

I would add a test, but the tests already fail in my Docker environment for the same reason, I'm not sure what exactly in the environment causes it.

Edit: I spent some time looking at the logic used to determine an encoding for open() when none is specified.
On a Unix system, without CODESET available, it looks for four environment variables 'LC_ALL', 'LC_CTYPE', 'LANG', 'LANGUAGE'. If none of those are set, it defaults to ascii. The container where the build fails has none of those four set.
While this is an environment issue, I still think it's worth changing the character, to match the other apostrophes in the file which all use the ASCII character.

root@9248effa1077:/data/datadogpy# python3.6 setup.py test
Traceback (most recent call last):
  File "setup.py", line 24, in <module>
    long_description=get_readme_md_contents(),
  File "setup.py", line 8, in get_readme_md_contents
    long_description = f.read()
  File "/usr/local/lib/python3.6/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 3377: ordinal not in range(128)

[zippolyte]

Thanks @Alphadash, LGTM