crashes when no aux file generated (with fix)

When no *.aux file is generated by pdflatex, the following error occurs:

Traceback (most recent call last):
  File "/usr/bin/rubber", line 9, in <module>
    sys.exit(Main()(sys.argv[1:]))
  File "/usr/lib/python2.7/site-packages/rubber/cmdline.py", line 296, in __call__
    return self.main(cmdline)
  File "/usr/lib/python2.7/site-packages/rubber/cmdline.py", line 260, in main
    ret = env.final.make(self.force)
  File "/usr/lib/python2.7/site-packages/rubber/__init__.py", line 237, in make
    ret = self.run()
  File "/usr/lib/python2.7/site-packages/rubber/rules/latex/__init__.py", line 1230, in run
    if self.compile(): return 1
  File "/usr/lib/python2.7/site-packages/rubber/rules/latex/__init__.py", line 1136, in compile
    self.aux_md5[aux] = md5_file(aux)
  File "/usr/lib/python2.7/site-packages/rubber/util.py", line 23, in md5_file
    file = open(fname)
IOError: [Errno 2] No such file or directory: 'myfile.aux'

The fix appears to be to change rules/latex/__init__.py line 1136 from

   self.aux_md5[aux] = md5_file(aux)

... to ...

   if exists(aux):
    self.aux_md5[aux] = md5_file(aux)
   else:
    self.aux_md5[aux] = None

I didn't deeply investigate the logic, but this seems to mirror usage elsewhere, where the md5 is set to None when the aux file doesn't exist.