Bump google-cloud-dns from 0.29.0 to 0.31.0

Bumps [google-cloud-dns](https://github.com/GoogleCloudPlatform/google-cloud-python) from 0.29.0 to 0.31.0.
- [Release notes](https://github.com/GoogleCloudPlatform/google-cloud-python/releases)
- [Changelog](https://github.com/googleapis/google-cloud-python/blob/master/CHANGELOG.md)
- [Commits](https://github.com/GoogleCloudPlatform/google-cloud-python/compare/0.29.0...0.31.0)

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requirements.txt

@@ -9,7 +9,7 @@ dyn==1.8.1
 edgegrid-python==1.1.1
 futures==3.2.0; python_version < '3.0'
 google-cloud-core==1.1.0
-google-cloud-dns==0.29.0
+google-cloud-dns==0.31.0
 ipaddress==1.0.22
 jmespath==0.9.4
 msrestazure==0.6.2