fix for python3 error when running python's 2 sitecustomize

When running python3 installed with brew, while also having brew's current python formula, I get an error:

```
Error in sitecustomize; set PYTHONVERBOSE for traceback:
ValueError: list.remove(x): x not in list
```

Setting PYTHONVERBOSE, I get:

```
  File "/usr/local/lib/python2.7/site-packages/sitecustomize.py", line 21, in <module>
    sys.path.remove('/usr/local/Cellar/python/2.7.3/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages')
ValueError: list.remove(x): x not in list
```

Since python3 doesn't have that path inside it's sys.path, the ValueError happens.