Fix import to work with pip version 18.1

It also works with previous versions of pip.

The alias to 'get_installed_distributions' in 'pip._internal' module
was removed in pip commit db72d427c5538a29df182bf7a724c304ac323cf6

Ref:
pypa/pip@db72d42

Fixes #105

pipdeptree.py

@@ -14,7 +14,7 @@
     from ordereddict import OrderedDict
 
 try:
-    from pip._internal import get_installed_distributions
+    from pip._internal.utils.misc import get_installed_distributions
     from pip._internal.operations.freeze import FrozenRequirement
 except ImportError:
     from pip import get_installed_distributions, FrozenRequirement