Fix jQuery validation on checkboxes in 'On behalf of' profiles

Should fix #298 (closed)

See also #241 and marketing/civicrm-website#253 (closed)

@kurund @tapash mattwire @danaskallman @andyburns @kcristiano are you able to test? Should work with both release 6.5.x branch and unreleased 6.6 branches.