setup: Add long_description_content_type (osquery#69)

To check this passes tests use:
  python setup.py sdist
  twine check dist/

Author: theopolis

.travis.yml

@@ -11,6 +11,9 @@ python:
   - pypy3
 install:
   - pip install -r requirements.txt
+  - pip install twine readme_renderer
 script:
   - python setup.py build
   - python setup.py test
+  - python setup.py sdist
+  - twine check dist/*

setup.py

@@ -78,6 +78,7 @@ def run(self):
       version=VERSION,
       description="Osquery Python API",
       long_description=README,
+      long_description_content_type="text/markdown",
       author=AUTHOR,
       author_email="osquery@fb.com",
       url="https://osquery.io",