Updated to last version available (3.1.3)
See https://pypi.python.org/simple/supervisor/
Corrected issue with "service supervisor restart" :
=> Failed to call refresh: Could not restart Service[supervisord]: Execution of '/etc/init.d/supervisord restart' returned 1: Restarting supervisord:

Used last version of init script available from https://github.com/Supervisor/initscripts/blob/master/ubuntu