Graylog 0.11.0 + rbenv

Switches to using rbenv, so this can be installed without destroying other ruby applications set up on the same box, and can use a newer version of ruby than whatever happens to be default on the machine. It will also build the passenger module itself, to avoid any conflicts with other modules.

This also incorporates PRs #15, $17, #18, #19, and upgrades Graylog2 to use 0.11.0.