Bump ffi-rzmq to 2.0.1

The [documentation for using ZeroMQ as an input](http://logstash.net/docs/1.4.0/inputs/zeromq) states that only version 2.1.x of ZeroMQ is supported. ZeroMQ is now on version 4 and by bumping the `ffi-rzmq` dependency we should be able to support the latest version of ZeroMQ as well.

I was able to make this work locally by replacing the `ffi-rzmq` directory in `vendor/bundle/jruby/1.9/gems` with `ffi-rzmq` 2.0.1 built on JRuby 1.7.9 using `jruby -S gem install ffi-rzmq. I also had to copy over `ffi-rzmq-core` but, core is a dependency of `ffi-rzmq` so as long as this gets bumped it should resolve that as well.