deps: update dependency com.google.cloud:google-cloud-bigtable-bom to v1.15.0 (#866)

This PR contains the following updates:

| Package | Update | Change |
|---|---|---|
| [com.google.cloud:google-cloud-bigtable-bom](https://github.com/googleapis/java-bigtable) | minor | `1.14.0` -> `1.15.0` |

---

### Release Notes

<details>
<summary>googleapis/java-bigtable</summary>

### [`v1.15.0`](https://github.com/googleapis/java-bigtable/blob/master/CHANGELOG.md#&#8203;1150-httpswwwgithubcomgoogleapisjava-bigtablecomparev1140v1150-2020-09-01)

[Compare Source](https://github.com/googleapis/java-bigtable/compare/v1.14.0...v1.15.0)

##### Features

-   extend channel priming logic to also send fake requests ([#&#8203;398](https://github.com/googleapis/java-bigtable/issues/398)) ([6f1ead2](https://github.com/googleapis/java-bigtable/commit/6f1ead2097150a87cb9712bcf35c6eaa9d57440c))
-   **deps:** adopt flatten plugin and google-cloud-shared-dependencies ([#&#8203;350](https://github.com/googleapis/java-bigtable/issues/350)) ([2298596](https://github.com/googleapis/java-bigtable/commit/2298596dab8a1ef87c0f48d3abe8bc3955417eb1))

##### Bug Fixes

-   temporarily disable reporting to unblock releases ([#&#8203;395](https://github.com/googleapis/java-bigtable/issues/395)) ([a56a0f8](https://github.com/googleapis/java-bigtable/commit/a56a0f8c9caf675b68d02587b042e1feeb261ccb))

##### Dependencies

-   update dependency com.google.cloud:google-cloud-shared-dependencies to v0.8.6 ([#&#8203;377](https://github.com/googleapis/java-bigtable/issues/377)) ([bdae336](https://github.com/googleapis/java-bigtable/commit/bdae336074d80815dcaaf8c71befafc0ed66c079))
-   update dependency com.google.cloud:google-cloud-shared-dependencies to v0.9.0 ([#&#8203;402](https://github.com/googleapis/java-bigtable/issues/402)) ([08f7d84](https://github.com/googleapis/java-bigtable/commit/08f7d84333c6a74bf03e0a57707b878a29400dd4))

</details>

---

### Renovate configuration

:date: **Schedule**: At any time (no schedule defined).

:vertical_traffic_light: **Automerge**: Disabled by config. Please merge this manually once you are satisfied.

:recycle: **Rebasing**: Renovate will not automatically rebase this PR, because other commits have been found.

:no_bell: **Ignore**: Close this PR and you won't be reminded about this update again.

---

 - [ ] <!-- rebase-check -->If you want to rebase/retry this PR, check this box

---

This PR has been generated by [WhiteSource Renovate](https://renovate.whitesourcesoftware.com). View repository job log [here](https://app.renovatebot.com/dashboard#github/googleapis/java-cloud-bom).

Author: renovate-bot

pom.xml

@@ -251,7 +251,7 @@
       <dependency>
         <groupId>com.google.cloud</groupId>
         <artifactId>google-cloud-bigtable-bom</artifactId>
-        <version>1.14.0</version>
+        <version>1.15.0</version>
         <type>pom</type>
         <scope>import</scope>
       </dependency>