deps: update dependency com.google.cloud:google-cloud-spanner-bom to v1.59.0 (#727)

This PR contains the following updates:

| Package | Update | Change |
|---|---|---|
| [com.google.cloud:google-cloud-spanner-bom](https://github.com/googleapis/java-spanner) | minor | `1.58.0` -> `1.59.0` |

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### Release Notes

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

### [`v1.59.0`](https://github.com/googleapis/java-spanner/blob/master/CHANGELOG.md#&#8203;1590-httpswwwgithubcomgoogleapisjava-spannercomparev1580v1590-2020-07-16)

[Compare Source](https://github.com/googleapis/java-spanner/compare/v1.58.0...v1.59.0)

##### Features

-   add support for NUMERIC data type ([#&#8203;193](https://github.com/googleapis/java-spanner/issues/193)) ([b38a91d](https://github.com/googleapis/java-spanner/commit/b38a91d8daac264b9dea327d6b31430d9599bd78))
-   spanner NUMERIC type ([#&#8203;349](https://github.com/googleapis/java-spanner/issues/349)) ([78c3192](https://github.com/googleapis/java-spanner/commit/78c3192266c474fc43277a8bf3f15caa968a0100))

##### Bug Fixes

-   check if emulator is running if env var is set ([#&#8203;340](https://github.com/googleapis/java-spanner/issues/340)) ([597f501](https://github.com/googleapis/java-spanner/commit/597f501803e6d58717a6e3770e6fd3f34454e9a5))
-   fix potential unnecessary transaction retry ([#&#8203;337](https://github.com/googleapis/java-spanner/issues/337)) ([1a4f4fd](https://github.com/googleapis/java-spanner/commit/1a4f4fd675a1580c87ad1d53c650a20bd2ff4811)), closes [#&#8203;327](https://github.com/googleapis/java-spanner/issues/327)
-   respect PDML timeout when using streaming RPC ([#&#8203;338](https://github.com/googleapis/java-spanner/issues/338)) ([d67f108](https://github.com/googleapis/java-spanner/commit/d67f108e86925c1296e695db8e78fa82e11fa4fa))
-   runs sample tests in java 8 and java 11 ([#&#8203;345](https://github.com/googleapis/java-spanner/issues/345)) ([b547e31](https://github.com/googleapis/java-spanner/commit/b547e31d095be3cf1646e0e9c07bfc467ecc3c22))
-   set gRPC keep-alive to 120 seconds ([#&#8203;339](https://github.com/googleapis/java-spanner/issues/339)) ([26be103](https://github.com/googleapis/java-spanner/commit/26be103da1117c4940550fad1672c66e6edfbdb3))

##### Dependencies

-   update dependency com.google.cloud:google-cloud-shared-dependencies to v0.8.3 ([#&#8203;334](https://github.com/googleapis/java-spanner/issues/334)) ([45acd89](https://github.com/googleapis/java-spanner/commit/45acd8960c961d48e91a7b1546efa64d9e9ae576))
-   update shared config to 0.9.2 ([#&#8203;328](https://github.com/googleapis/java-spanner/issues/328)) ([75df62c](https://github.com/googleapis/java-spanner/commit/75df62c0176137fda1d0a9076b83be06f11228ce))

</details>

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pom.xml

@@ -526,7 +526,7 @@
       <dependency>
         <groupId>com.google.cloud</groupId>
         <artifactId>google-cloud-spanner-bom</artifactId>
-        <version>1.58.0</version>
+        <version>1.59.0</version>
         <type>pom</type>
         <scope>import</scope>
       </dependency>