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.
