Post-release cleanup for language and google-cloud. (#4329)

In particular:

- Marking the package versions as "dev"
- Updating the `releases.rst` for the packages

(Again, this showcases a "need" for the `releases.rst` files to
be checked in some automated way.)

packages/google-cloud-language/setup.py

@@ -60,7 +60,7 @@
 
 setup(
     name='google-cloud-language',
-    version='0.31.0',
+    version='0.31.1.dev1',
     description='Python Client for Google Cloud Natural Language',
     long_description=README,
     namespace_packages=[