Improve virtualenv extension: handle vim-pyenv

[vim-pyenv](https://github.com/lambdalisue/vim-pyenv) handles pyenv's
virtualenvs / Pythons.

/cc @lambdalisue
the original statement was correct.  `VIRTUAL_ENV` is the final fallback if other plugins are not available so it should be included in the check.  also, not sure why you included `airline_enable_virtualenv`, as that's more or less deprecated from over a year ago and all new extensions should follow the `#` style only.