Changes in pip-10.0.0b2 break pipdeptree

[sourcenouveau]

pipdeptree is broken with pip-10.0.0b2.

$ pipdeptree -j
Traceback (most recent call last):
  File "/home/someone/venv/bin/pipdeptree", line 11, in <module>
    sys.exit(main())
  File "/home/someone/venv/lib/python2.7/site-packages/pipdeptree.py", line 550, in main
    pkgs = pip.get_installed_distributions(local_only=args.local_only,
AttributeError: 'module' object has no attribute 'get_installed_distributions'

I believe this is due to the following item in the change notes for pip:

Move all of pip's APIs into the pip._internal package, properly reflecting the fact that pip does not currently have any public APIs. (#4696, #4700)

[naiquevin]

Yes, I am aware of the refactoring change (See #83). Will push a fix in a day or two. PR is also welcome. Thanks 👍