Update an import for PyLint 1.0

In PyLint 1.0, released on Aug 6, the IASTNGChecker class was renamed to IAstroidChecker. I made a fallback import to work with old or new name, but preferring new name going forward.

commit that changed name:
https://bitbucket.org/logilab/pylint/src/bdcbb695e59a/interfaces.py