Limit pytest <4.0.0

Some of our tests fail with the new pytest v4.0.x versions because of
a change to how fixtures work. Since these versions are the latest
release in PyPI now, unit tests will fail on a fresh install.

This change limits the version range in setup.py to fix the issue
until we can make a decision about upgrading.

ml-agents/setup.py

         'matplotlib',
         'numpy>=1.13.3,<=1.14.5',
         'jupyter',
-        'pytest>=3.2.2',
+        'pytest>=3.2.2,<4.0.0',
         'docopt',
         'pyyaml',
         'protobuf>=3.6,<3.7',