Ruby 1.8 rand fixes

Fixes the issue with making calls to rand for Ruby 1.8.  Basically the same end result as https://github.com/kzk/unicorn-worker-killer/pull/10 but this should merge in cleaner.

If you want to just go with the other pull request, no hard feelings.  I just figured I'd put out an easy drop in fix so that people like myself can get back to using the canonical gem on Ruby 1.8 projects.