import OrderedDict from git.odict rather than directly from collections, to pix Py2.6 compatibility

[boegel]

cfr. https://github.com/gitpython-developers/GitPython/pull/431/files/89ade7bfff534ae799d7dd693b206931d5ed3d4f#r64649040

cc @Byron

I tested this in my fork on Travis; I'm running into one failing test, but it seems unrelated to this change.
Am I missing something to make the tests pass?

======================================================================
ERROR: test_init_repo_object (git.test.test_docs.Tutorials)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/travis/build/boegel/GitPython/git/ext/gitdb/gitdb/test/lib.py", line 87, in wrapper
    return func(self, path)
  File "/home/travis/build/boegel/GitPython/git/test/test_docs.py", line 68, in test_init_repo_object
    assert repo.refs['origin/master'] == repo.remotes.origin.refs.master  # ... remotes ...
  File "/home/travis/build/boegel/GitPython/git/util.py", line 684, in __getitem__
    raise IndexError("No item found with id %r" % (self._prefix + index))
IndexError: No item found with id 'origin/master'

[Byron]

The unit-tests are not perfectly isolated, and being on a branch makes some of them fail. It's a known issue, also one that I am not proud of :).
Travis is happy, here you go, thanks for your contribution !

[boegel]

Thanks @Byron!