Cutting version 0.28.0 of google-cloud-core. (#4280)

Also

- updating all dependencies of `grpcio` to `>= 1.7.0`. This was due to an issue [1] with `1.6.0`.
- updating the version of `google-api-core` (also to be released, This is required since the bounds on `grpcio` of `google-cloud-core==0.28.0` and `google-api-core==0.1.0` are mutually exclusive.)
- Updating `google-api-core` CHANGELOG for release.
- Updating packages to depend on `google-cloud-core>=0.28.0`.
- Installing `nox -s lint` deps locally for vision.

[1]: grpc/grpc#12455

packages/google-cloud-vision/nox.py

@@ -117,7 +117,7 @@ def lint(session):
     serious code quality issues.
     """
     session.interpreter = 'python3.6'
-    session.install('flake8', 'pylint', 'gcp-devrel-py-tools')
+    session.install('flake8', 'pylint', 'gcp-devrel-py-tools', *LOCAL_DEPS)
     session.install('.')
     session.run('flake8', 'google/cloud/vision')
     session.run(

packages/google-cloud-vision/setup.py

@@ -25,7 +25,7 @@
     readme = readme_file.read()
 
 REQUIREMENTS = [
-    'google-cloud-core[grpc] >= 0.27.1, < 0.28dev',
+    'google-cloud-core[grpc] >= 0.28.0, < 0.29dev',
     'google-gax >= 0.15.14, < 0.16dev',
 ]
 EXTRAS_REQUIRE = {