Preparing for a release of all packages.

Towards #2441.

- Updating umbrella README to point at all packages
- Putting upper bounds on grpcio in dependencies
- Putting lower bounds on all google-cloud-* packages
  listed as dependencies
- Adding `setup.cfg` for universal wheels

packages/google-cloud-language/setup.py

@@ -50,12 +50,12 @@
 
 
 REQUIREMENTS = [
-    'google-cloud-core',
+    'google-cloud-core >= 0.20.0',
 ]
 
 setup(
     name='google-cloud-language',
-    version='0.20.0dev',
+    version='0.20.0',
     description='Python Client for Google Cloud Natural Language',
     long_description=README,
     namespace_packages=[