Have you tried running that job from the command-line, as the user
whose crontab the job is executed?
If that works, it has to be an environmental issue. For example, if
you're using Gentoo Linux, interactive shells will have RUBYOPT=-
rauto_gem in the environment, which causes rubygems to be required
automatically, for any ruby script. However, that environment
variable isn't set in a cron job.