Bypass the missing jid files

The most recent release 2014.1.1 has a little logic issue while cleaning old jobs.
At exactly 65 seconds after the master started, one of the master processes get defunct.

Process Process-1:
Traceback (most recent call last):
  File "/usr/lib64/python2.7/multiprocessing/process.py", line 258, in _bootstrap
    self.run()
  File "/usr/lib64/python2.7/multiprocessing/process.py", line 114, in run
    self._target(*self._args, **self._kwargs)
  File "/usr/lib64/python2.7/site-packages/salt/master.py", line 246, in _clear_old_jobs
    with salt.utils.fopen(jid_file, 'r') as fn_:
  File "/usr/lib64/python2.7/site-packages/salt/utils/__init__.py", line 1037, in fopen
    fhandle = open(*args, **kwargs)
IOError: [Errno 2] No such file or directory: '/var/cache/salt/master/jobs/44/9e352281c9938b05de853cdfb96ea0/jid'

This PR fixes this issue.