Bump protobuf-java from 3.9.0 to 3.9.1 (#167)

Bumps [protobuf-java](https://github.com/protocolbuffers/protobuf) from 3.9.0 to 3.9.1.
- [Release notes](https://github.com/protocolbuffers/protobuf/releases)
- [Changelog](https://github.com/protocolbuffers/protobuf/blob/master/generate_changelog.py)
- [Commits](protocolbuffers/protobuf@v3.9.0...v3.9.1)

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

examples/java/build.gradle

@@ -53,7 +53,7 @@ dependencies {
 
     compile 'org.apache.commons:commons-math3:3.6.1'
 
-    compile 'com.google.protobuf:protobuf-java:3.9.0'
+    compile 'com.google.protobuf:protobuf-java:3.9.1'
 
     compile 'io.grpc:grpc-netty:1.22.1'
     compile 'io.grpc:grpc-protobuf:1.22.1'