Require Ruby 1.9.3 or greater moving forward.

After the next patch release of engineyard 2.3.x, I'll be maintaining engineyard gem only on 1.9.3. The 2.3.x branch will continue to live on 2-3-stable and will receive updates if they impact user security.

This will mainly affect users of Ruby 1.8.7. The engineyard gem does not need to load any of your application code, so development can continue on ruby 1.8.7, if necessary, while running ey deploy from 1.9.3.