Fix crash with requests 2.5 where connectionpool was None

Was getting the following crash with requests 2.5. Didn't dig deep enough to determine if this is the appropriate fix, but it seems to work.

```
Traceback (most recent call last):
  File "C:\Python27\Lib\unittest\case.py", line 327, in run
    testMethod()
  File "C:\Users\Chase\projects\flexget\py27\lib\site-packages\nose\case.py", line 197, in runTest
    self.test(*self.arg)
  File "C:\Users\Chase\projects\flexget\tests\__init__.py", line 98, in func_with_cassette
    func(*args, **kwargs)
  File "C:\Users\Chase\projects\flexget\py27\lib\site-packages\vcr\cassette.py", line 59, in __exit__
    next(self.__finish, None)
  File "C:\Users\Chase\projects\flexget\py27\lib\site-packages\vcr\cassette.py", line 50, in _patch_generator
    cassette._save()
  File "C:\Users\Chase\projects\flexget\py27\lib\site-packages\contextlib2.py", line 257, in __exit__
    return _invoke_next_callback(exc_details)
  File "C:\Users\Chase\projects\flexget\py27\lib\site-packages\contextlib2.py", line 244, in _invoke_next_callback
    suppress_exc = _invoke_next_callback(exc_details)
  File "C:\Users\Chase\projects\flexget\py27\lib\site-packages\contextlib2.py", line 244, in _invoke_next_callback
    suppress_exc = _invoke_next_callback(exc_details)
  File "C:\Users\Chase\projects\flexget\py27\lib\site-packages\contextlib2.py", line 244, in _invoke_next_callback
    suppress_exc = _invoke_next_callback(exc_details)
  File "C:\Users\Chase\projects\flexget\py27\lib\site-packages\contextlib2.py", line 244, in _invoke_next_callback
    suppress_exc = _invoke_next_callback(exc_details)
  File "C:\Users\Chase\projects\flexget\py27\lib\site-packages\contextlib2.py", line 244, in _invoke_next_callback
    suppress_exc = _invoke_next_callback(exc_details)
  File "C:\Users\Chase\projects\flexget\py27\lib\site-packages\contextlib2.py", line 244, in _invoke_next_callback
    suppress_exc = _invoke_next_callback(exc_details)
  File "C:\Users\Chase\projects\flexget\py27\lib\site-packages\contextlib2.py", line 244, in _invoke_next_callback
    suppress_exc = _invoke_next_callback(exc_details)
  File "C:\Users\Chase\projects\flexget\py27\lib\site-packages\contextlib2.py", line 244, in _invoke_next_callback
    suppress_exc = _invoke_next_callback(exc_details)
  File "C:\Users\Chase\projects\flexget\py27\lib\site-packages\contextlib2.py", line 244, in _invoke_next_callback
    suppress_exc = _invoke_next_callback(exc_details)
  File "C:\Users\Chase\projects\flexget\py27\lib\site-packages\contextlib2.py", line 244, in _invoke_next_callback
    suppress_exc = _invoke_next_callback(exc_details)
  File "C:\Users\Chase\projects\flexget\py27\lib\site-packages\contextlib2.py", line 244, in _invoke_next_callback
    suppress_exc = _invoke_next_callback(exc_details)
  File "C:\Users\Chase\projects\flexget\py27\lib\site-packages\contextlib2.py", line 244, in _invoke_next_callback
    suppress_exc = _invoke_next_callback(exc_details)
  File "C:\Users\Chase\projects\flexget\py27\lib\site-packages\contextlib2.py", line 254, in _invoke_next_callback
    suppress_exc = cb(*exc_details) or suppress_exc
  File "C:\Users\Chase\projects\flexget\py27\lib\site-packages\contextlib2.py", line 171, in _exit_wrapper
    return cm_exit(cm, *exc_details)
  File "C:\Users\Chase\projects\flexget\py27\lib\site-packages\vcr\patch.py", line 239, in __exit__
    while not pool.pool.empty() and connections:
AttributeError: 'NoneType' object has no attribute 'empty'
```