deps: update core dependencies to v1.54.0 (#68)

This PR contains the following updates:

| Package | Update | Change |
|---|---|---|
| [com.google.api:gax-grpc](https://togithub.com/googleapis/gax-java) | minor | `1.53.1` -> `1.54.0` |
| [com.google.api:gax-bom](https://togithub.com/googleapis/gax-java) | minor | `1.53.1` -> `1.54.0` |

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### Release Notes

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

### [`v1.54.0`](https://togithub.com/googleapis/gax-java/releases/v1.54.0)

[Compare Source](https://togithub.com/googleapis/gax-java/compare/v1.53.1...v1.54.0)

02-27-2020 09:07 PST

##### New Features

-   Scale number of threads to scale with number of cpus ([#&#8203;878](https://togithub.com/googleapis/gax-java/pull/878))
-   Do not shut down executor when WatchDog is shut down ([#&#8203;871](https://togithub.com/googleapis/gax-java/pull/871))

##### Dependencies

-   Update google-http-client to 1.34.2 ([#&#8203;886](https://togithub.com/googleapis/gax-java/pull/886))
-   Update gRPC to 1.27.2 ([#&#8203;885](https://togithub.com/googleapis/gax-java/pull/885))
-   Update protobuf to 3.11.4 and gRPC to 1.27.1 ([#&#8203;882](https://togithub.com/googleapis/gax-java/pull/882))

##### Documentation

-   Add javadoc about being called within single thread ([#&#8203;856](https://togithub.com/googleapis/gax-java/pull/856))

##### Internal / Testing Changes

-   Update jacoco to 0.8.5 ([#&#8203;883](https://togithub.com/googleapis/gax-java/pull/883))
-   Update google format plugin ([#&#8203;868](https://togithub.com/googleapis/gax-java/pull/868))
-   Set up linkage monitor Kokoro build config for gax-java ([#&#8203;864](https://togithub.com/googleapis/gax-java/pull/864))

</details>

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pom.xml

@@ -66,7 +66,7 @@
     <google.core.version>1.91.1</google.core.version>
     <google.api-common.version>1.8.1</google.api-common.version>
     <google.common-protos.version>1.17.0</google.common-protos.version>
-    <gax.version>1.53.1</gax.version>
+    <gax.version>1.54.0</gax.version>
     <grpc.version>1.27.2</grpc.version>
     <protobuf.version>3.11.4</protobuf.version>
     <junit.version>4.13</junit.version>