Updating all affected packages after google-cloud-core update. (#3730)

* Updating all affected packages after google-cloud-core update.

* Moving 'pip install .' **after** subpackages in nox docs.

@lukesneeringer still hasn't explained why it was moved. In
it's current location, the depencencies are first retrieved
from PyPI (which fails here for the unreleased versions), e.g.

https://circleci.com/gh/GoogleCloudPlatform/google-cloud-python/2716

Author: dhermes

packages/google-cloud-spanner/setup.py

@@ -51,7 +51,7 @@
 
 
 REQUIREMENTS = [
-    'google-cloud-core >= 0.25.0, < 0.26dev',
+    'google-cloud-core >= 0.26.0, < 0.27dev',
     'grpcio >= 1.2.0, < 2.0dev',
     'gapic-google-cloud-spanner-v1 >= 0.15.0, < 0.16dev',
     'gapic-google-cloud-spanner-admin-database-v1 >= 0.15.0, < 0.16dev',
@@ -60,7 +60,7 @@
 
 setup(
     name='google-cloud-spanner',
-    version='0.25.0',
+    version='0.26.0',
     description='Python Client for Cloud Spanner',
     long_description=README,
     namespace_packages=[