Update Sinatra to 1.4.4

This contains a fix for https://github.com/sinatra/sinatra/issues/739 that
will make problems with slow downloads from iPXE disappear.

The root cause of the problem was that Sinatra's send_file would not set a
Content-Length header which forced iPXE into a very slow mode of
downloading files, which particularly hurt with the initrd. Now, Sinatra
sets Content-Length properly and iPXE downloads reasonably fast.