chore: regenerate README (#415)

This PR was generated using Autosynth. 🌈


<details><summary>Log from Synthtool</summary>

```
2020-10-30 23:36:39,890 synthtool [DEBUG] > Executing /root/.cache/synthtool/java-pubsub/.github/readme/synth.py.
On branch autosynth-readme
nothing to commit, working directory clean
2020-10-30 23:36:40,845 synthtool [DEBUG] > Wrote metadata to .github/readme/synth.metadata/synth.metadata.

```
</details>

Full log will be available here:
https://source.cloud.google.com/results/invocations/7e9c5ad1-2dee-4027-8b0b-ff3ea24defcb/targets

- [ ] To automatically regenerate this PR, check this box.

.github/readme/synth.metadata/synth.metadata

@@ -4,7 +4,7 @@
       "git": {
         "name": ".",
         "remote": "https://github.com/googleapis/java-pubsub.git",
-        "sha": "d4115871c7ad9f9168f6c144264ec96e210a9857"
+        "sha": "ac3fa8dce86b40d4845682f1e37d609d5e64fc80"
       }
     },
     {

README.md

@@ -17,7 +17,7 @@ If you are using Maven with [BOM][libraries-bom], add this to your pom.xml file
     <dependency>
       <groupId>com.google.cloud</groupId>
       <artifactId>libraries-bom</artifactId>
-      <version>13.3.0</version>
+      <version>13.4.0</version>
       <type>pom</type>
       <scope>import</scope>
     </dependency>