Actually clear GEM_PATH

This helps ensure we provide only our own gem directory for rubygems.

Helps improve LOGSTASH-1918. Before this patch, my local ruby install
was accidentally providing 'rack' so I never noticed this problem.
Add 'rack' gem back in

This is required for 'logstash web' to serve Kibana and was
removed accidentally.

This fixes LOGSTASH-1918
Add test coverage for LOGSTASH-1918
Merge pull request #1164 from jordansissel/LOGSTASH-1918

Add missing 'rack' gem