Update dependency google-cloud-spanner to v1.15.1 (#3377)

This PR contains the following updates:

| Package | Update | Change |
|---|---|---|
| [google-cloud-spanner](https://togithub.com/googleapis/python-spanner) | patch | `==1.15.0` -> `==1.15.1` |

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### Release Notes

<details>
<summary>googleapis/python-spanner</summary>

### [`v1.15.1`](https://togithub.com/googleapis/python-spanner/blob/master/CHANGELOG.md#&#8203;1151-httpswwwgithubcomgoogleapispython-spannercomparev1150v1151-2020-04-08)

[Compare Source](https://togithub.com/googleapis/python-spanner/compare/v1.15.0...v1.15.1)

</details>

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appengine/standard_python37/spanner/requirements.txt

@@ -1,2 +1,2 @@
-google-cloud-spanner==1.15.0
+google-cloud-spanner==1.15.1
 Flask==1.1.1

functions/spanner/requirements.txt

@@ -1 +1 @@
-google-cloud-spanner==1.15.0
+google-cloud-spanner==1.15.1

spanner/cloud-client/requirements.txt

@@ -1,2 +1,2 @@
-google-cloud-spanner==1.15.0
+google-cloud-spanner==1.15.1
 futures==3.3.0; python_version < "3"