LOGSTASH-1553: Docs: Changed all dead links to the flatjar to a working url

See issue https://logstash.jira.com/browse/LOGSTASH-1553
I changed all the dead links to the URL used on http://logstash.net.
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Yesterday I created pull request https://github.com/logstash/logstash/pull/755 as well. I'm not sure where this belongs.