Require Ruby 1.9.3 or greater for next major release

Wait to merge until after next release of 2.3.x.

After the next patch release of engineyard 2.3.x, engineyard gem will only be maintained for >= ruby-1.9.3. The 2.3.x branch will continue on the 2-3-stable branch and will receive updates if they impact user security.

This will mainly affect users of Ruby 1.8.7. However, the engineyard gem does not need to run any of your application code. Development can continue on ruby 1.8.7, if necessary, while running ey deploy from ruby 1.9.3.