- make 'target' an absolute path otherwise ruby 2.0.0+rubygems 2.x
  refuses to install things. Cute :(
- Disable doc generation on rubygems 2
- Try to set GEM_HOME/GEM_PATH early on in the process.
- make RUBYVER tunable for ruby 2.0, not that it works well yet.
Merge git://github.com/logstash/logstash