Pin google-protobuf to v3.11.x, to fix CI

CI message from `bundle install`:
```
google-protobuf-3.12.0-x86_64-linux requires ruby version >= 2.5, which is
incompatible with the current version, ruby 2.4.6p354
```

protocolbuffers/protobuf#7453

fluent-plugin-gcloud-pubsub-custom.gemspec

@@ -17,7 +17,11 @@ Gem::Specification.new do |gem|
   gem.require_paths = ["lib"]
 
   gem.add_runtime_dependency "fluentd", [">= 0.14.15", "< 2"]
+
   gem.add_runtime_dependency "google-cloud-pubsub", "~> 0.30.0"
+  # google-protobuf does not support Ruby 2.4 since v3.12.0, which we test against in CI.
+  # https://github.com/protocolbuffers/protobuf/pull/7453
+  gem.add_runtime_dependency "google-protobuf", "~> 3.11.4"
 
   gem.add_development_dependency "bundler"
   gem.add_development_dependency "rake"