Bump com.google.protobuf:protobuf-gradle-plugin from 0.9.3 to 0.9.4 (#153)

Bumps [com.google.protobuf:protobuf-gradle-plugin](https://github.com/google/protobuf-gradle-plugin) from 0.9.3 to 0.9.4.
- [Release notes](https://github.com/google/protobuf-gradle-plugin/releases)
- [Commits](https://github.com/google/protobuf-gradle-plugin/commits)

---
updated-dependencies:
- dependency-name: com.google.protobuf:protobuf-gradle-plugin
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <support@github.com>

java-spiffe-core/build.gradle

@@ -4,7 +4,7 @@ buildscript {
     }
 
     dependencies {
-        classpath group: 'com.google.protobuf', name: 'protobuf-gradle-plugin', version: '0.9.3'
+        classpath group: 'com.google.protobuf', name: 'protobuf-gradle-plugin', version: '0.9.4'
     }
 }