fixing bug: AttributeError: DumpMaster instance has no attribute 'unload...

Without this fix, I kept getting this error when using mitmdump -s bla.py -r somefile:

Traceback (most recent call last):
  File "/usr/local/bin/mitmdump", line 55, in <module>
    m.run()
  File "/usr/local/lib/python2.7/dist-packages/libmproxy/dump.py", line 227, in run
    self.unload_script(script)
AttributeError: DumpMaster instance has no attribute 'unload_script'