deps: update core dependencies to v1.56.0 (#258)

This PR contains the following updates:

| Package | Update | Change |
|---|---|---|
| [com.google.api:gax-grpc](https://togithub.com/googleapis/gax-java) | minor | `1.55.0` -> `1.56.0` |
| [com.google.api:gax-bom](https://togithub.com/googleapis/gax-java) | minor | `1.55.0` -> `1.56.0` |

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### Release Notes

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

### [`v1.56.0`](https://togithub.com/googleapis/gax-java/releases/v1.56.0)

[Compare Source](https://togithub.com/googleapis/gax-java/compare/v1.55.0...v1.56.0)

04-06-2020 10:29 PDT

##### New Features

-   Add ability to partially update retry settings ([#&#8203;993](https://togithub.com/googleapis/gax-java/pull/993))

##### Dependencies

-   Update grpc to 1.28.1 ([#&#8203;1013](https://togithub.com/googleapis/gax-java/pull/1013))
-   Update com.google.api:api-common to 1.9.0 ([#&#8203;992](https://togithub.com/googleapis/gax-java/pull/992))
-   Add javax.annotations dependency in bazel build to gax-grpc ([#&#8203;1000](https://togithub.com/googleapis/gax-java/pull/1000))

##### Internal / Testing Changes

-   test: make it easier to debug failures in SettingsTest ([#&#8203;994](https://togithub.com/googleapis/gax-java/pull/994))
-   build: update dependency gradle.plugin.com.dorongold.plugins:task-tree to v1.5 ([#&#8203;995](https://togithub.com/googleapis/gax-java/pull/995))
-   test: update dependency com.google.api.grpc:grpc-google-cloud-pubsub-v1 to v1.86.0 ([#&#8203;1008](https://togithub.com/googleapis/gax-java/pull/1008))

</details>

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pom.xml

@@ -66,7 +66,7 @@
     <google.core.version>1.93.3</google.core.version>
     <google.api-common.version>1.9.0</google.api-common.version>
     <google.common-protos.version>1.17.0</google.common-protos.version>
-    <gax.version>1.55.0</gax.version>
+    <gax.version>1.56.0</gax.version>
     <grpc.version>1.28.1</grpc.version>
     <protobuf.version>3.11.4</protobuf.version>
     <junit.version>4.13</junit.version>