Reduce numpy version in ml-agents-envs setup (#4274)

ml-agents-envs/setup.py

     install_requires=[
         "cloudpickle",
         "grpcio>=1.11.0",
-        "numpy>=1.14.1,<2.0",
+        "numpy>=1.14.1,<1.19.0",
         "Pillow>=4.2.1",
         "protobuf>=3.6",
         "pyyaml>=3.1.0",