Bug 1105421 we shouldnt install prodtxt

@bhearsum r?

It works like this:
```
(balrog)peterbe@Peters-MacBook-Pro:~/dev/MOZILLA/BALROG/balrog (bug-1105421-we-shouldnt-install-prodtxt *+)$ python
Python 2.6.9 (unknown, Sep  9 2014, 15:05:12)
[GCC 4.2.1 Compatible Apple LLVM 6.0 (clang-600.0.39)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import blinker
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ImportError: No module named blinker
>>> import thirdparty
>>> thirdparty.extendsyspath()
>>> import blinker
>>> blinker.__file__
'/Users/peterbe/dev/MOZILLA/BALROG/balrog/vendor/lib/python/blinker/__init__.pyc'
>>>
```

I also tried installing a different version of `blinker` in my virtualenv and after doing the `thirdparty.extendsyspath()` trick is made sure that the `blinker` that is imported is the one in `vendor/lib/python`.
This all looks fine, but can you put this into auslib.util to avoid cluttering up the top level?