Support installing plugin with MRI

When using USE_RUBY=1 with a non-jruby I got the following errors trying to install plugins

1. unresolved require of jar-dependencies => fixed by modifying the gemspec for non-jruby rubies
2. undefined **java** constant in  lib/logstash/pluginmanager/util.rb => proposed fix is to use **Dir.tmpdir()** 

@electrical waiting for your review