add twine check in tox (#651)

Author: ChoiByungWook

tox.ini

@@ -4,7 +4,8 @@
 # and then run "tox" from this directory.
 
 [tox]
-envlist = py27,py35,flake8,pylint,sphinx
+envlist = py27,py35,flake8,pylint,twine,sphinx
+
 skip_missing_interpreters = False
 
 [travis]
@@ -73,6 +74,17 @@ deps =
 commands =
     python -m pylint --rcfile=.pylintrc -j 0 src/sagemaker
 
+[testenv:twine]
+basepython = python
+# twine check was added starting in 1.12.0
+# https://github.com/pypa/twine/blob/master/docs/changelog.rst
+deps =
+    twine>=1.12.0
+# https://packaging.python.org/guides/making-a-pypi-friendly-readme/#validating-restructuredtext-markup
+commands =
+    python setup.py sdist
+    twine check dist/*.tar.gz
+
 [testenv:sphinx]
 basepython = python3
 changedir = doc