Update logstash.spec

We have been keeping this file up to date for our needs.

I updated all the files as it is now it will compile and work on CentOS, Fedora and RHEL, I use this both for work and on my private fedora repository: www.ulyaoth.net

We removed the "plugins" as with 1.2.0 and upwards the plugins folder cannot be empty and thus breaking logstash.