Do not check for when VIRTUALENVWRAPPER_LOG_DIR

Executing virtualenvwrapper hooks was protected by this check. I have
no idea why.
Use print function instead of statement.

This actually broke virtualenvwrapper functionality for Python 3.
Add pyvenv-tracking-mode to track the current virtualenv.

When this mode is active, pyvenv will adjust the current virtualenv
to the buffer-locally specified one after every command, adjusting
the current virtualenv every time the buffer changes.