Make version-checking compatible with updated Packaging library.

[avnishks]

Recently the packaging library removed the LegacyVersion class after nearly two years of deprecation warning [1]. This has resulted in Surfa's setup.py script throwing the following error at installation time:
AttributeError: module 'packaging.version' has no attribute 'LegacyVersion'

Proposed fix:
Instead of checking if the (parsed) version string is a LegacyVersion object, use the packaging.version.parse(version) function to parse the version string and catch any InvalidVersion exception. If an exception is raised, then its not a valid version string according to the PEP 440 specification.

[1] pypa/packaging#530

[pombredanne]

FYI, I created https://github.com/nexB/packvers/ to cope with the issues of pypa/packaging#530 in https://github.com/nexB/pip-requirements-parser and https://github.com/nexB/scancode-toolkit and a few more projects