deps: update core dependencies to v1.93.4 (#90)

This PR contains the following updates:

| Package | Update | Change |
|---|---|---|
| [com.google.cloud:google-cloud-core](https://togithub.com/googleapis/java-core) | patch | `1.93.3` -> `1.93.4` |
| [com.google.cloud:google-cloud-core-http](https://togithub.com/googleapis/java-core) | patch | `1.93.3` -> `1.93.4` |

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### Release Notes

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

### [`v1.93.4`](https://togithub.com/googleapis/java-core/blob/master/CHANGELOG.md#&#8203;1934-httpswwwgithubcomgoogleapisjava-corecomparev1933v1934-2020-04-06)

[Compare Source](https://togithub.com/googleapis/java-core/compare/v1.93.3...v1.93.4)

</details>

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pom.xml

@@ -152,7 +152,7 @@
     <github.global.server>github</github.global.server>
     <site.installationModule>google-cloud-datastore-parent</site.installationModule>
     <project.javadoc.protobufBaseURL>https://googleapis.dev/java/google-api-grpc/latest</project.javadoc.protobufBaseURL>
-    <google.core.version>1.93.3</google.core.version>
+    <google.core.version>1.93.4</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>1.56.0</gax.version>