Fix maven enforcer plugin warning

Fix for the following maven warning

```bash
$ mvn install
[INFO] Scanning for projects...
[WARNING] 
[WARNING] Some problems were encountered while building the effective model for com.google.cloud.genomics:readstore-local-java:jar:v1beta
[WARNING] 'build.plugins.plugin.version' for org.apache.maven.plugins:maven-enforcer-plugin is missing. @ line 112, column 21
[WARNING] 
[WARNING] It is highly recommended to fix these problems because they threaten the stability of your build.
[WARNING] 
[WARNING] For this reason, future Maven versions might no longer support building such malformed projects.
[WARNING]
...
```