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

Author: dhermes

setup.py

@@ -50,16 +50,16 @@
 
 
 REQUIREMENTS = [
-    'google-cloud-core',
-    'grpcio >= 1.0.0',
+    'google-cloud-core >= 0.20.0',
+    'grpcio >= 1.0.0, < 2.0dev',
     'google-gax >= 0.14.1, < 0.15dev',
     'gapic-google-pubsub-v1 >= 0.9.0, < 0.10dev',
     'grpc-google-pubsub-v1 >= 0.9.0, < 0.10dev',
 ]
 
 setup(
     name='google-cloud-pubsub',
-    version='0.20.0dev',
+    version='0.20.0',
     description='Python Client for Google Cloud Pub/Sub',
     long_description=README,
     namespace_packages=[