Propose upgrading client to Beta [skip ci]. (#959)

All the bugs for the milestone are closed.

Author: coryan

README.md

@@ -20,7 +20,7 @@ C++ Idiomatic Clients for [Google Cloud Platform][cloud-platform] services.
 [client-library-docs]: http://GoogleCloudPlatform.github.io/google-cloud-cpp/
 
 This library supports the following Google Cloud Platform services with clients
-at an [Alpha](#versioning) quality level:
+at the [Beta](#versioning) quality level:
 
 - [Google Cloud Bigtable](google/cloud/bigtable#google-cloud-bigtable)
 

google/cloud/bigtable/README.md

@@ -12,7 +12,7 @@ client.
 ## Status
 
 This library support Google Cloud Bigtable at the
-[Alpha](../README.md#versioning) quality level.
+[Beta](../README.md#versioning) quality level.
 
 ## Documentation