The current list of directories to search for autoenv on misses the default location on Ubuntu systems if you just do a normal `pip install autoenv` - [it will place](https://github.com/kennethreitz/autoenv/blob/master/setup.py#L16) `activate.sh` in `/usr/local/bin` unless you manually override the `--prefix` or something.
The `/usr/local/opt/autoenv` is fine for macOS/homebrew installations but it would be nice not to have to manually patch on Linux :)
Sources the activate.sh file or informs the user, that its not
available.
Overriding the cd function, as it was formerly done, is not needed
anymore, activate.sh does this by itself.
Marc Cornellà