Disable resolv-replace for ruby 1.9.3

@see conflict with spork https://github.com/sporkrb/spork/issues/133

Not sure this should be merged as is, maybe you should investigate further what is the issue with resolv-replace rather than disabling it completely for ruby 1.9.3.

I see this is made for performance purpose, but I think overwriting standard libs should not be made without consent from the gem user (maybe you could a configuration option for this).