chore: regenerate README (#257)

This PR was generated using Autosynth. 🌈


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

```
2020-11-05 23:05:08,600 synthtool [DEBUG] > Executing /root/.cache/synthtool/java-billing/.github/readme/synth.py.
On branch autosynth-readme
nothing to commit, working directory clean
2020-11-05 23:05:09,506 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/7f8b84af-9c9f-4e45-b2e5-dea32eedb30d/targets

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

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

@@ -4,14 +4,14 @@
       "git": {
         "name": ".",
         "remote": "https://github.com/googleapis/java-billing.git",
-        "sha": "7e8a0e406ad808ad6e596bf51b4f4049a339728c"
+        "sha": "17651784af64efe0aee6c9c99cb603389c581157"
       }
     },
     {
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "ba9918cd22874245b55734f57470c719b577e591"
+        "sha": "2c8aecedd55b0480fb4e123b6e07fa5b12953862"
       }
     }
   ]

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>13.4.0</version>
+      <version>15.0.0</version>
       <type>pom</type>
       <scope>import</scope>
     </dependency>