Googleads

* Updated PyYAML dependencies to >=4.2b1 for https://nvd.nist.gov/vuln/detail/CVE-2017-18342

* Added Smart Shopping ad example (v201809)

* Added support for v201811.
* Removed support for v201711.
* Removed examples for v201802.
* Remove ContentMetadataKeyHierarchyService
* Added version parameter to PQL utilities to account for statement
differences among API versions. This parameter defaults to the most recent
API version. If you update your Python client library but don't plan on
migrating to the newest API version, you might need to explicitly declare the
version for the PQL utility to use.

* Added support and examples for AdWords v201809.
* Resolved issues:
- Issue 281: https://github.com/googleads/googleads-python-lib/issues/281

* DFP rebranded to Google Ad Manager.
* Google Ad Manager v201808 release.
* Removed support for DFP v201708.
* Removed examples for DFP v201711.

* Removed support for AdWords v201710.