Beginning with the Ubuntu package jack-audio-connection-kit-0.118+svn3796-1ubuntu2 editing of /etc/security/limits.conf should not be required. JACK will write the following during post-installation to /etc/security/limits.d/audio.conf

# generated by jackd's postinst.
# Do not edit this file by hand, use
# dpkg-reconfigure -p high jack
# instead
@audio - rtprio 99
@audio - memlock unlimited
#@audio - nice -19

Please note that creating or modifying /etc/security/limits.conf may cause conflicts with the JACK written /etc/security/limits.d/audio.conf settings.