Fix #369 relative command path

The docs say:

    The command can be either absolute (e.g. /path/to/programname) or relative (e.g. programname). 
    If it is relative, the supervisord’s environment $PATH will be searched for the executable.

This fixes relative path like to/programname as stated in #369 .
Yes, but `'/' in program does` not mean it IS NOT a relative path(or IS absolute). Only `program.startswith('/')` means absolute path? If so, this code doesn't not comply with the docs.