Logstash plugins should target the Java platform.

Since plugins cannot work without the `logstash-core-java-event` it makes sense that we make sure the plugins are publishing only as java gems.

This mean we need to add to all the plugins gemspec the following line `s.platform = "java"`.

This will also prevent people to do a bundle install under MRI, since bundler will yield a useful message telling them they need to have the correct ruby platform to proceed.