Only define `deactivate` alias when virtual env is active
Only define `deactivate` alias when virtual env is active

Typing the first few letters of `deactivate` now gives clear indication
that there is no active virtual environment, even if one didn't notice
the virtual environment name missing from the shell prompt.

The underlying `vf deactivate` command will continue to be available at
all times.

adambrenecki/virtualfish,adambrenecki/virtualfish