Updating install_requires to be more strict.

docs/Installation.md

 To install dependencies, enter the `ml-agents/` directory and run from
 the command line:
 
-    pip install -r requirements.txt
     pip install .
 
 ## Docker-based Installation

ml-agents/setup.py

     packages=find_packages(exclude=['tests', 'tests.*', '*.tests', '*.tests.*']),  # Required
 
     install_requires=[
-        'tensorflow>=1.7',
+        'tensorflow>=1.7,<1.8',
         'Pillow>=4.2.1',
         'matplotlib',
         'numpy>=1.13.3,<=1.14.5',
         'pyyaml',
-        'protobuf>=3.6.0',
+        'protobuf>=3.6,<3.7',
         'grpcio>=1.11.0'],
 
     entry_points={