Raise error if ruby 1.9.3 is detected.

@mperham, Let me know if i need to reformulate the error message.

I'm not sure this should go here (where it will cause Rails server to fail to start also) or in cli.rb (where it will just kill the sidekiq server).  wdyt?

What about Rubinius or other future VMs?  I think it might be better to explicitly look for MRI, rather than exclude java.