fix: stop leaking sockets on restart (affects ruby 1.9.3 or before)

Using puma on ruby 1.9.3, when I restart the server by using `kill -USR2`, it doesn't close sockets and therefore it leaks sockets.  On ruby 2.0.0 this leak problem doesn't happen.

This is because ruby 2.0 `Kernel#exec` has its option `:close_others` implicitly set to `true`.  Any version before 2.0 should have this option explicitly.

I'll be sending a  reproducing code for this problem later.