Make it work on RHEL6

RHEL6 ships with an older version of Ruby (and besides that, hardcoding the
interpreter as "1.9.9" does not make sense). In addition, this older version of
rubygems apparently requires one to explicitly import that module before one can
use the systemwide gems.

If you're interested in RHEL6 packaging, see https://build.opensuse.org/package/show/home:jkt-gentoo:vesnicky-el6/puavo-tftpd .