Prepend @ to variable names in template to avoid deprecation warnings

Hello,

I made this change to avoid all the messages like the following
```
(at /tmp/vagrant-puppet-1/modules-0/supervisord/templates/program.conf.erb:29:in `block in result')
Warning: Variable access via 'stderr_events_enabled' is deprecated. Use '@stderr_events_enabled' instead. template[/tmp/vagrant-puppet-1/modules-0/supervisord/templates/program.conf.erb]:30
```
when using your plugin in puppet 3.4.3.

regards,

Yves-Marie Haussonne