Add missing import to fix OrderedDict.clear.

`OrderedDict.clear` crashes on Python 2.6. See [this](https://travis-ci.org/obspy/obspy/jobs/43326824) for example:
```
 File "/home/travis/virtualenv/python2.6.9/lib/python2.6/site-packages/requests/packages/urllib3/_collections.py", line 89, in clear
self._container.clear()
File "/home/travis/virtualenv/python2.6.9/lib/python2.6/site-packages/future/backports/misc.py", line 113, in clear
for node in itervalues(self.__map):

NameError: global name 'itervalues' is not defined
```

I _think_ all that needs to be done is to add the `itervalues` import to `future/backports/misc.py`, but I don't have a Py2.6 install to check. We'll see what happens on Travis...