Change 'with' statement in utils/platforms.py to a 'try' clause

ZipFile only gained the '__exit__' method at python 2.7, which is used by the 'with' statement to ensure a graceful exit of the wrapped code. Since we're using 2.6, this wouldn't work at that version. Ported said code to use a 'try-except' clause to make it compatible with our target version.