Only chsh to zsh if zsh is not the shell

This PR checks the user's current shell before making the chsh call.  This is a good thing because the chsh call requires a password input, so it'd be nice to avoid that where possible.
Just tested this on my OS X machine:

```bash
if [[ "$(bash -c 'echo $SHELL')" != "/bin/zsh" ]]; then
  fancy_echo "Changing your shell to zsh ..."
    chsh -s $(which zsh)
fi
```

And it worked as expected. I think that follows our style guide, too?