Update dependency google-auth to v1.6.1 (#2769)

This PR contains the following updates:

| Package | Update | Change | References |
|---|---|---|---|
| google-auth | minor | `==1.5.1` -> `==1.6.1` | [source](https://renovatebot.com/gh/GoogleCloudPlatform/google-auth-library-python) |

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### Release Notes

<details>
<summary>GoogleCloudPlatform/google-auth-library-python</summary>

### [`v1.6.1`](https://renovatebot.com/gh/GoogleCloudPlatform/google-auth-library-python/blob/master/CHANGELOG.rst#v161)

[Compare Source](https://renovatebot.com/gh/GoogleCloudPlatform/google-auth-library-python/compare/v1.6.0...v1.6.1)

11-12-2018 10:10 PST

Implementation Changes
\++++++++++++++++++++++

-   Automatically refresh impersonated credentials (`#&#8203;304 <https://github.com/googleapis/google-auth-library-python/pull/304>`\_)

### [`v1.6.0`](https://renovatebot.com/gh/GoogleCloudPlatform/google-auth-library-python/blob/master/CHANGELOG.rst#v160)

[Compare Source](https://renovatebot.com/gh/GoogleCloudPlatform/google-auth-library-python/compare/v1.5.1...v1.6.0)

11-09-2018 11:07 PST

New Features
\++++++++++++

-   Add google.auth.impersonated_credentials (`#&#8203;299 <https://github.com/googleapis/google-auth-library-python/pull/299>`_)

Documentation
\+++++++++++++

-   Update link to documentation for default credentials (`#&#8203;296 <https://github.com/googleapis/google-auth-library-python/pull/296>`\_)
-   Update github issue templates (`#&#8203;300 <https://github.com/googleapis/google-auth-library-python/pull/300>`\_)
-   Remove punctuation which becomes part of the url (`#&#8203;284 <https://github.com/googleapis/google-auth-library-python/pull/284>`\_)

Internal / Testing Changes
\++++++++++++++++++++++++++

-   Update trampoline.sh (`#&#8203;302 <https://github.com/googleapis/google-auth-library-python/pull/302>`\_)
-   Enable static type checking with pytype (`#&#8203;298 <https://github.com/googleapis/google-auth-library-python/pull/298>`\_)
-   Make classifiers in setup.py an array. (`#&#8203;280 <https://github.com/googleapis/google-auth-library-python/pull/280>`\_)

</details>

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frozen_requirements.txt

@@ -52,7 +52,7 @@ entrypoints==0.2.3
 flake8==3.6.0
 future==0.17.1
 google-api-python-client==1.7.4
-google-auth==1.5.1
+google-auth==1.6.1
 google-auth-httplib2==0.0.3
 httplib2==0.11.3
 identify==1.1.7